SUMMARY

        This summary highlights selected information included in this document and does not contain all of the information that may be important to you. You should read this entire document and its appendices and the other documents to which the parties refer before you decide how to vote with respect to the proposals. In addition, the parties incorporate by reference important business and financial information about Square 1 and PacWest into this document. For a description of this information, please see the section entitled "Incorporation of Certain Documents by Reference." You may obtain the information incorporated by reference into this document without charge by following the instructions in the section entitled "Where You Can Find More Information" in the forepart of this document. Each item in this summary includes a page reference directing you to a more complete description of that item.

        Unless the context otherwise requires, throughout this document, "PacWest" refers to PacWest Bancorp, "Square 1" refers to Square 1 Financial, Inc. and "we," "us" and "our" refer collectively to PacWest and Square 1. Also, the parties refer to the proposed merger of Square 1 with and into PacWest as the "merger," the proposed merger of Square 1 Bank with and into Pacific Western Bank as the "bank merger" and the Agreement and Plan of Merger, dated as of March 1, 2015, by and between PacWest and Square 1 as the "merger agreement."

The Merger and the Merger Agreement (pages 41 and 72)

        The terms and conditions of the merger are contained in the merger agreement, which is attached to this document as Appendix A. The parties encourage you to read the merger agreement carefully, as it is the legal document that governs the merger.

        Under the terms of the merger agreement, Square 1 will merge with and into PacWest, with PacWest being the surviving corporation.

Merger Consideration (page 73)

        In the merger, each share of Square 1 common stock, par value $0.01 per share, owned by a Square 1 stockholder will be converted into the right to receive 0.5997 of a share of PacWest common stock, par value $0.01 per share. For each fractional share that would otherwise be issued, PacWest will pay cash in an amount equal to the fraction of a share of PacWest common stock which the holder would otherwise be entitled to receive multiplied by the PacWest Average Closing Price. No interest will be paid or accrue on cash payable to holders in lieu of fractional shares.

        The share price of PacWest common stock will fluctuate, and the value that holders of Square 1 common stock will receive upon consummation of the merger may be different than the value that they would receive if calculated on the date PacWest and Square 1 publicly announced the signing of the merger agreement, on the date that this document is being mailed to Square 1 stockholders, and on the date of the special meeting. Based on the closing price of PacWest common stock on February 27, 2015, the last trading day before the public announcement of the signing of the merger agreement, the value of the per share merger consideration payable to holders of Square 1 common stock was $27.49. Based on the closing price of PacWest common stock on [    ], 2015, the last practicable date before the date of this document, the value of the per share merger consideration payable to holders of Square 1 common stock was $[    ].

Recommendation of the Square 1 Board of Directors (page 46)

        After careful consideration, the Square 1 board of directors recommends that Square 1 stockholders vote "FOR" the merger proposal and "FOR" the adjournment proposal.

        Certain stockholders of Square 1 have entered into voting agreements with PacWest, pursuant to which each such stockholder has agreed to vote "FOR" the merger proposal and "FOR" any other

matter required to be approved by the stockholders of Square 1 to facilitate the transactions contemplated by the merger agreement. For more information regarding the voting agreements, please see the section entitled "The Merger Agreement—Voting Agreements."

        For a more complete description of Square 1's reasons for the merger and the recommendations of the Square 1 board of directors, please see the section entitled "The Merger—Recommendation of the Square 1 Board of Directors and Reasons for the Merger."

Opinion of Financial Advisor (page 48)

        At the March 1, 2015 meeting at which the board of directors of Square 1 considered and approved the merger agreement, Sandler O'Neill & Partners, L.P., referred to as Sandler O'Neill, delivered to the board its oral opinion, which was subsequently confirmed in writing, that, as of such date, subject to procedures followed, assumptions made, matters considered and qualifications and limitations described in Sandler O'Neill's opinion, the merger consideration was fair to the holders of Square 1 common stock from a financial point of view.

        The full text of Sandler O'Neill's opinion is attached as Appendix C to this proxy statement/prospectus. The opinion outlines the procedures followed, assumptions made, matters considered and qualifications and limitations on the review undertaken by Sandler O'Neill in rendering its opinion. Holders of Square 1 common stock are urged to read the entire opinion carefully in connection with their consideration of the proposed merger.

        Sandler O'Neill's opinion speaks only as of the date of the opinion and was necessarily based on financial, economic, market and other conditions as they existed on, and the information made available to Sandler O'Neill as of, that date. The opinion was directed to the board of directors of Square 1 and is directed only to the fairness of the merger consideration to the holders of Square 1 common stock from a financial point of view. It does not address the underlying business decision of Square 1 to engage in the merger or any other aspect of the merger and is not a recommendation to any holder of Square 1 common stock as to how such stockholder should vote at the special meeting with respect to the merger or any other matter. Sandler O'Neill did not express any opinion as to the fairness of the amount or nature of the compensation to be received in the merger by Square 1's officers, directors or employees, or class of such persons, relative to the per share consideration to be received by Square 1 stockholders.

        For further information, please see the section entitled "The Merger—Opinion of Square 1's Financial Advisor."

Square 1 Special Meeting (page 34)

        The special meeting will be held at [    ], local time, on [    ], 2015, at [    ], located at [    ]. At the special meeting, holders of Square 1 Class A common stock will be asked to approve the merger proposal and the adjournment proposal and to act upon other business as may properly come before the special meeting or any adjournment or postponement thereof by or at the direction of the Square 1 board of directors.

        The Square 1 board of directors has fixed the close of business on [    ], 2015 as the record date for determining the holders of Square 1 Class A common stock entitled to receive notice of and to vote at the special meeting. As of the close of business on the record date, there were [    ] shares of Square 1 Class A common stock outstanding and entitled to vote at the special meeting held by [    ] holders of record. Each share of Square 1 Class A common stock entitles the holder to one vote on each proposal to be considered at the special meeting. As of the close of business on the record date, directors and executive officers of Square 1 and their affiliates owned and were entitled to vote [    ] shares of Square 1 Class A common stock, representing approximately [    ]% of the shares of Square 1 Class A

common stock outstanding on that date. Certain stockholders of Square 1 have entered into voting agreements with PacWest, pursuant to which they have agreed, solely in their capacity as stockholders of Square 1, to vote all of their shares of Square 1 Class A common stock "FOR" the merger proposal and any other matter required to be approved by the stockholders of Square 1 to facilitate the transactions contemplated by the merger agreement. As of the close of business on the record date, these stockholders beneficially owned, in the aggregate, [    ] shares of Square 1 Class A common stock, allowing them to exercise approximately [    ]% of the voting power of Square 1 common stock. As of close of business on the record date, PacWest beneficially held no shares of Square 1 common stock.

        Approval of the merger proposal requires the affirmative vote of a majority of the outstanding shares of Square 1 Class A common stock entitled to vote on such proposal. Approval of the adjournment proposal requires the affirmative vote of a majority of the votes cast at the special meeting.

Square 1's Directors and Executive Officers Have Certain Interests in the Merger (page 63)

        In considering the recommendation of the Square 1 board of directors with respect to the merger, Square 1 Class A common stockholders should be aware that the executive officers and directors of Square 1 have certain interests in the merger that may be different from, or in addition to, the interests of Square 1 stockholders generally. The Square 1 board of directors was aware of these interests and considered them, among other matters, in approving the merger agreement and the transactions contemplated thereby and making its recommendation that Square 1 Class A common stockholders vote to approve the Square 1 merger proposal. These interests include:

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outstanding Square 1 stock options and RSUs (including those held by the directors and executive officers) will vest upon consummation of the merger and be settled in cash based on the approximate value of the merger consideration (less, in the case of stock options, the applicable exercise price);



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letter agreements between PacWest and each of Douglas Bowers, Samir Bhaumik, Diane Earle, Judith Erwin and Frank Tower provide for certain retention awards and other benefits upon consummation of the merger;



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employment agreements between Square 1 and each of Jason Kranack, Patrick Oakes and Gregory Thompson provide for severance benefits upon certain qualifying terminations of employment following the consummation of the merger;



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any employee of Square 1 (including executive officers) who experiences a severance qualifying termination of employment following consummation of the merger will be eligible for an annual bonus for 2015, provided that in the event the termination date occurs during 2015, such bonus will be prorated for the portion of 2015 that elapsed prior to the date of termination; and



•

Square 1's directors and executive officers are entitled to continued indemnification and insurance coverage under the merger agreement.

        We estimate that the aggregate amount that would become payable to Square 1's eight executive officers in settlement of their unvested Square 1 stock options and RSUs, as severance benefits (including a prorated bonus for 2015) and in respect of retention awards due upon the closing of the merger, assuming that the closing of the merger occurred on May 31, 2015 and each executive officer experienced a severance-qualifying termination at such time, is $14,422,331. In addition, we estimate that the aggregate amount of the retention awards payable to Mr. Bowers, Mr. Bhaumik, Ms. Earle, Ms. Erwin and Mr. Tower in respect of post-closing services is $3,435,000. We estimate that the aggregate amount that would become payable to Square 1's 11 non-employee directors in settlement of their unvested Square 1 stock options and RSUs is $679,359. The foregoing amounts do not include certain interests that cannot be quantified at this time, such as continued indemnification and

compensation that might be paid in respect of continued employment. For a more complete description of these interests, and information regarding how the foregoing amounts were estimated, see "The Merger—Interests of Square 1 Directors and Executive Officers in the Merger."

Treatment of Square 1 Stock Options and Square 1 RSUs (page 41)

        Square 1 Stock Options.    At the effective time, each outstanding Square 1 stock option, whether vested or unvested, will be cancelled and will entitle the holder of such option to receive an amount in cash equal to the product of (i) the total number of shares of Square 1 common stock subject to such option and (ii) the excess, if any, of (A) the product of (1) the PacWest Average Closing Price and (2) 0.5997 over (B) the exercise price per share of Square 1 common stock underlying such option, less any applicable taxes to be withheld with respect to such payment.

        Square 1 RSUs.    At the effective time, each outstanding Square 1 RSU will be cancelled and will entitle the holder to receive an amount in cash equal to the product of (i) the number of shares of Square 1 common stock subject to such Square 1 RSU and (ii) the product of (A) the PacWest Average Closing Price and (B) 0.5997, less any applicable taxes to be withheld with respect to such payment.

Treatment of Square 1 Warrants (page 42)

        At the effective time, each warrant to purchase shares of Square 1 common stock held by specified individuals to the extent then outstanding and not previously exercised will be cancelled and will only entitle the holder thereof to receive, as soon as reasonably practicable after the effective time, an amount in cash equal to the product of (i) the total number of shares of Square 1 common stock subject to such warrants and (ii) the excess, if any, of (A) the product of (1) the PacWest Average Closing Price and (2) 0.5997 over (B) the exercise price per share of Square 1 common stock under such warrants, less any applicable taxes required to be withheld with respect to such payment. As of the date of this document, there were outstanding warrants to purchase [    ] shares of Square 1 common stock. All such warrants expire, if not previously exercised, on July 11, 2015.

Board of Directors and Officers of PacWest After the Merger (page 63)

        On or prior to the effective time, the PacWest board of directors will increase the number of directors constituting the PacWest board of directors by one and fill such vacancy by a current director of Square 1 selected by Square 1 and reasonably acceptable to PacWest to be effective immediately following the effective time. The officers of PacWest immediately prior to the effective time will be the officers of the surviving corporation until the earlier of their resignation or removal or until their respective successors are duly appointed and qualified.

Regulatory Approvals Required for the Merger (page 68)

        Completion of the merger and the bank merger are subject to various regulatory approvals, including approvals from the Federal Reserve Board, the FDIC, the CDBO and the CBNC. Notifications and/or applications requesting approval for the merger or for the bank merger may also be submitted to other federal and state regulatory authorities and self-regulatory organizations. The parties have filed notices and applications to obtain the necessary regulatory approvals of the Federal Reserve Board, the FDIC, the CDBO and the CBNC. Although the parties currently believe they should be able to obtain all required regulatory approvals in a timely manner, they cannot be certain when or if they will obtain them or, if obtained, whether they will contain terms, conditions or restrictions not currently contemplated that will be detrimental to or have a material adverse effect on PacWest after the completion of the merger. The regulatory approvals to which completion of the

merger and bank merger are subject are described in more detail under the section entitled "The Merger—Regulatory Approvals Required for the Merger."

Conditions to the Merger (page 86)

        The obligations of PacWest and Square 1 to complete the merger are each subject to the satisfaction or waiver of the following conditions:

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approval of the merger proposal by the Square 1 stockholders;



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no governmental authority of competent jurisdiction having enacted, issued, promulgated, enforced or entered any law (whether temporary, preliminary or permanent) that is in effect and restrains, enjoins or otherwise prohibits consummation of the merger, the bank merger or the other transactions contemplated by the merger agreement; and



•

the S-4 registration statement, of which this document is a part, having become effective under the Securities Act of 1933, as amended, referred to as the Securities Act, no stop order suspending the effectiveness of the S-4 registration statement having been issued, and no proceedings for that purpose having been initiated or been threatened, by the SEC.

        In addition, the obligation of Square 1 to complete the merger is subject to the satisfaction or waiver of the following conditions:

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the accuracy of the representations and warranties of PacWest set forth in the merger agreement, subject to the materiality standards set forth in the merger agreement, as of the date of the merger agreement and as of the closing date of the merger as though made at and as of the closing date (except that representations and warranties that by their terms speak as of the date of the merger agreement or some other date need only be true and correct as of such date), and Square 1's receipt of a certificate signed on behalf of PacWest by an executive officer of PacWest, dated as of the closing date, to such effect;



•

performance by PacWest in all material respects of all obligations required to be performed by it under the merger agreement at or prior to the effective time, and receipt by Square 1 of a certificate signed by an executive officer of PacWest, dated as of the closing date, to such effect;



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receipt by Square 1 of the opinion of its counsel, dated the closing date, to the effect that, on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify for U.S. federal income tax purposes as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code;



•

all consents, registrations, approvals, permits and authorizations required to be obtained prior to the effective time by either party or any of its respective subsidiaries from the Federal Reserve Board, the FDIC, the CDBO and the CBNC which are necessary to consummate the merger and the bank merger, and any other consents, registrations, approvals, permits and authorizations from or with any governmental authority the failure of which to be obtained is reasonably likely to have, individually or in the aggregate, a material adverse effect on Square 1 or PacWest (measured on a scale relative to Square 1), having been made or obtained (as the case may be) and remaining in full force and effect and all statutory waiting periods in respect thereof having expired; and



•

since March 1, 2015, no event having occurred or circumstance arisen that, individually or taken together with all other facts, circumstances or events, has had or is reasonably likely to have a material adverse effect with respect to PacWest.

        PacWest's obligation to complete the merger is also subject to the satisfaction or waiver of the following conditions:

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the accuracy of the representations and warranties of Square 1 set forth in the merger agreement, subject to the materiality standards set forth in the merger agreement, as of the date of the merger agreement and as of the closing date of the merger as though made at and as of the closing date (except that representations and warranties that by their terms speak as of the date of the merger agreement or some other date need only be true and correct as of such date) and PacWest's receipt of a certificate signed on behalf of Square 1 by an executive officer of Square 1, dated as of the closing date, to such effect;



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performance by Square 1 in all material respects of all obligations required to be performed by it under the merger agreement at or prior to the effective time, and receipt by PacWest of a certificate signed by an executive officer of Square 1, dated as of the closing date, to such effect;



•

all consents, registrations, approvals, permits and authorizations required to be obtained prior to the effective time by either party or any of its respective subsidiaries from the Federal Reserve Board, the FDIC, the CDBO and the CBNC which are necessary to consummate the merger and the bank merger, all consents, registrations, approvals, permits and authorizations required to be obtained prior to the effective time from any governmental authority that are required for the operation of Square 1's trust business and any other consents, registrations, approvals, permits and authorizations from or with any governmental authority the failure of which to be obtained is reasonably likely to have, individually or in the aggregate, a material adverse effect on Square 1 or PacWest (measured on a scale relative to Square 1), having been made or obtained (as the case may be) and remaining in full force and effect and all statutory waiting periods in respect thereof having expired, and none of such consents, registrations, approvals, permits and authorizations containing any materially burdensome regulatory condition;



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as of the last business day of the month reflected in the closing financial statements, the adjusted Square 1 stockholders' equity (defined below) will not be less than $302,704,000 as determined in accordance with GAAP; "adjusted Square 1 stockholders' equity" means the consolidated equity of Square 1 as set forth in the closing financial statements, minus any unrealized gains or plus any unrealized losses in Square 1's securities portfolio due to mark-to-market adjustments as of the last business day of the month reflected in the closing financial statements, and adding the sum of (i) all amounts paid or accrued in connection with certain actions taken to conform certain policies and practices to those of PacWest to the extent that such actions were not necessary to bring Square 1 into conformity with GAAP or any applicable law of any governmental authority, (ii) all fees and expenses of all attorneys, accountants, investment bankers and other advisors and agents for Square 1 for services rendered solely in connection with the transactions contemplated by the merger agreement paid by Square 1 prior to the effective time and which do not exceed in the aggregate $11,000,000 (exclusive of reasonable costs incurred or advanced by such advisors) and (iii) any amounts payable by Square 1 in connection with the cancellation of Square 1's equity awards pursuant to the merger agreement, and any costs associated with the termination of employee benefit plans or programs (inclusive of any severance compensation paid or to be paid as provided in the merger agreement) or any retention bonuses paid in accordance with the merger agreement, and any costs incurred in connection with the termination of Square 1's contracts pursuant to the merger agreement;



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receipt by PacWest of the opinion of its counsel, dated the closing date, to the effect that, on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify for U.S. federal income tax purposes as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code;

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since March 1, 2015, no event having occurred or circumstance arisen that, individually or taken together with all other facts, has had or is reasonably likely to have a material adverse effect with respect to Square 1; and



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receipt by PacWest of a certificate certifying that interests in Square 1 are not U.S. real property interests under the Internal Revenue Code.

No Solicitation (page 84)

        Under the terms of the merger agreement, Square 1 has agreed not to initiate, solicit, encourage or otherwise facilitate any inquiries or the making of any proposal or offer with respect to, or engage in any negotiations concerning, or provide any confidential information or data to, or have any discussions with, any person relating to, any acquisition proposal. Notwithstanding these restrictions, the merger agreement provides that, under specified circumstances, in response to an unsolicited bona fide written acquisition proposal from a third party, if the Square 1 board of directors determines in good faith (after consultation with its outside legal counsel) that the failure to take such action would reasonably be expected to violate the Square 1 directors' fiduciary duties under applicable law, Square 1 may (i) execute a confidentiality agreement with such third party to provide confidential information or (ii) engage in discussions or negotiations with such third party. The merger agreement also provides that, under specified circumstances, in response to an unsolicited bona fide written acquisition proposal if the Square 1 board of directors determines in good faith (i) after consultation with its financial advisors, that such proposal is a superior proposal and (ii) after consultation with outside legal counsel, that the failure to change its recommendation to the Square 1 stockholders to adopt the merger agreement would result in a violation of the Square 1 board of directors' fiduciary duties under applicable law, the Square 1 board of directors may change its recommendation to the Square 1 stockholders that the Square 1 stockholders adopt the merger agreement.

Termination of the Merger Agreement (page 88)

        PacWest and Square 1 may mutually agree at any time to terminate the merger agreement without completing the merger, even if the Square 1 stockholders have adopted the merger agreement.

        The merger agreement may also be terminated and the merger may be abandoned:

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at any time prior to the effective time, whether before or after the Square 1 stockholder approval, by action of the board of directors of either party, in the event that the merger is not consummated by March 31, 2016, except to the extent that the failure of the merger to be consummated results from the knowing action or inaction of the party seeking to terminate, which action or inaction is in violation of its obligations under the merger agreement;



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at any time prior to the effective time, whether before or after the Square 1 stockholder approval, by action of the board of directors of either party if:



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the approval of any governmental authority required for consummation of the merger, the bank merger or the other transactions contemplated by the merger agreement has been denied by final and nonappealable action of such governmental authority, or an application therefor has been permanently withdrawn by mutual agreement of the parties at the request or suggestion of a governmental authority; or



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the Square 1 stockholder approval is not obtained at the duly convened special meeting;



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at any time prior to the effective time, whether before or after the Square 1 stockholder approval, by action of either party's board if there has been a breach of any representation, warranty, covenant or agreement made by the other party, such that if continuing on the closing date of the merger, the condition as to the accuracy of the representations and warranties or the

compliance with covenants by the other party would not be satisfied and such breach or condition is not curable or, if curable, is not cured within 30 days after written notice thereof is given by the terminating party (or such shorter period as remaining prior to March 31, 2016); provided, that the terminating party is not then in material breach of any representation, warranty, covenant or agreement;

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by action of PacWest's board of directors at any time prior to the Square 1 stockholder approval, in the event:



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Square 1 has breached in any material respect the prohibitions in the merger agreement relating to acquisition proposals;



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the Square 1 board of directors has effected a change of recommendation;



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at any time after the end of 15 business days following receipt of an acquisition proposal, the Square 1 board of directors has failed to reaffirm its recommendation that Square 1 stockholders adopt the merger agreement as promptly as practicable (but in any event within five business days) after receipt of any written request to do so by PacWest; or



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a tender offer or exchange offer for outstanding shares of Square 1 common stock has been publicly disclosed (other than by PacWest or an affiliate of PacWest) and the Square 1 board of directors recommends that Square 1 stockholders tender their shares in such tender or exchange offer or, within 10 business days after the commencement of such tender or exchange offer, the Square 1 board of directors fails to recommend unequivocally against acceptance of such offer.

        For more information, please see the section entitled "The Merger Agreement—Termination of the Merger Agreement."

Termination Fee (page 89)

        Square 1 must pay PacWest a termination fee of $32.5 million in the following circumstances:

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PacWest terminates the merger agreement because: (i) Square 1 has breached in any material respect its obligations relating to acquisition proposals under the merger agreement, (ii) the Square 1 board of directors has changed its recommendation, (iii) any time after the end of 15 business days following receipt of an acquisition proposal, the Square 1 board of directors has failed to reaffirm its recommendation after receipt of a written request from PacWest, or (iv) a tender offer or exchange offer for outstanding shares of Square 1 common stock has been publicly disclosed and the Square 1 board of directors recommends that the Square 1 stockholders tender their shares or, within ten business days after the commencement of such tender or exchange offer, the Square 1 board of directors fails to recommend unequivocally against acceptance of such an offer; or



•

(1) (a) an acquisition proposal has been made to Square 1 or its stockholders generally or (b) any person has publicly announced an intention to make an acquisition proposal with respect to Square 1, (2) thereafter the merger agreement is terminated by either party because (a) the merger was not consummated on or before March 31, 2016 and the Square 1 stockholder approval has not been obtained or (b) the Square 1 stockholder approval was not obtained at the special meeting, and (3) within 12 months after the termination of the merger agreement, Square 1 enters into a definitive agreement with respect to or consummates certain acquisition proposals.

Voting Agreements (page 89)

        Patriot Financial Partners, L.P., Castle Creek Capital Partners IV, L.P., Endicott Opportunity Partners III, L.P. and Northaven Management, Inc. (and certain affiliates thereof), in their capacities as stockholders of Square 1, have separately entered into voting agreements with PacWest in which they have agreed to vote all shares of Square 1 common stock that they own and have the power to vote in favor of the merger proposal and any other matter that is required to be approved by the stockholders of Square 1 to facilitate the transactions contemplated by the merger agreement. These stockholders also agreed to vote against any proposal made in opposition to adoption of the merger agreement or in competition with the merger agreement and against any acquisition proposal. As of the close of business on the record date, these stockholders beneficially owned, in the aggregate, [    ] shares of Square 1 Class A common stock, allowing them to exercise approximately [    ]% of the voting power of Square 1 common stock (which does not include shares issuable upon the exercise of stock options that were not outstanding as of the close of business on the record date).

        The voting agreements terminate in certain circumstances, including in the event that the merger agreement is terminated in accordance with its terms.

        For more information, please see the section entitled "The Merger Agreement—Voting Agreements."

Material U.S. Federal Income Tax Consequences of the Merger (page 91)

        The merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code. Assuming the merger qualifies as a reorganization, a stockholder of Square 1 generally will not recognize any gain or loss upon receipt of PacWest common stock in exchange for Square 1 common stock in the merger (except for any gain or loss that may result from the receipt of cash in lieu of fractional shares of PacWest common stock that the stockholder of Square 1 would otherwise be entitled to receive). It is a condition to the completion of the merger that PacWest and Square 1 receive written opinions from their respective counsel to the effect that the merger will qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

        Tax matters are complicated and the tax consequences of the merger to each Square 1 stockholder may depend on such stockholder's particular facts and circumstances. Square 1 stockholders are urged to consult their tax advisors to understand fully the tax consequences to them of the merger. For more information, please see the section entitled "Material U.S. Federal Income Tax Consequences of the Merger."

Litigation Related to the Merger (page 90)

        Three putative stockholder class action lawsuits, referred to as the merger litigation, have been filed in connection with the merger agreement. The first action, Manganaro v. Burke et al., Case No. 10817-VCL, was filed on March 20, 2015 in the Court of Chancery of the State of Delaware. This action alleged, among other things, that the members of the Square 1 board of directors breached their fiduciary duties to Square 1 stockholders, and was voluntarily dismissed without prejudice on May 5, 2015.

        Two more actions were filed in the United States District Court for the Middle District of North Carolina: (1) Lakowitz v. Bowers et al., Case No. 1:15-CV-371, filed on May 6, 2015; and (2) Li v. Bowers et al., Case No. 1:15-CV-373, filed on May 7, 2015. Both complaints allege, on behalf of a putative class of Square 1 stockholders, that the members of the Square 1 board of directors breached their fiduciary duties to Square 1 stockholders by approving the proposed merger for inadequate consideration; approving the transaction in order to obtain benefits not equally shared by other Square 1 stockholders; engaging a conflicted financial advisor; entering into the merger agreement containing

preclusive deal protection devices; and causing a materially deficient and misleading Registration Statement to be filed and disseminated to stockholders. The complaints also allege claims against PacWest and Square 1 for aiding and abetting these alleged breaches of fiduciary duties. The plaintiffs in both actions also bring claims in their individual capacities against Square 1 and the members of the Square 1 board of directors for alleged violations of Sections 14(a) of the Securities Exchange Act of 1934 (the "Exchange Act"), and against the members of the Square 1 board of directors for alleged violations of Section 20(a) of the Exchange Act, based upon alleged omissions and misrepresentations in the Registration Statement. The plaintiffs in these actions seek injunctive relief prohibiting consummation of the merger, rescission and damages in the event the merger is consummated, an accounting of damages suffered by the plaintiffs and the putative class, attorneys' fees and costs, and other and further relief. At this stage, it is not possible to predict the outcome of the proceedings or their impact on Square 1, PacWest or the merger.

Comparison of Stockholders' Rights (page 94)

        The rights of Square 1 stockholders who continue as PacWest stockholders after the merger will be governed by the certificate of incorporation and bylaws of PacWest rather than by the certificate of incorporation and bylaws of Square 1. For more information, please see the section entitled "Comparison of Stockholders' Rights."

The Parties (page 39)

PacWest Bancorp
10250 Constellation Blvd., Suite 1640
Los Angeles, California 90067
Phone: (310) 286-1144

        PacWest Bancorp, a Delaware corporation, is a bank holding company registered under the Bank Holding Company Act of 1956, as amended. PacWest's principal business is to serve as the holding company for PacWest's wholly owned subsidiary, Pacific Western Bank. Pacific Western Bank is focused on relationship-based business banking to small and middle-market businesses nationwide. PacWest offers a broad range of deposit products and services through 80 full-service branches located throughout California. PacWest, through offices across the United States, provides loans to middle-market businesses, real estate investment firms, and sophisticated individual real estate investors.

        As of March 31 2015, PacWest had total assets of $16.6 billion, gross loans and leases of $12.3 billion, total deposits of $11.9 billion and stockholders' equity of $3.5 billion. PacWest's corporate headquarters is located in Los Angeles, California, and PacWest has 80 full-service, retail bank branches located primarily in southern and central California and three branches in northern California. PacWest's loan origination efforts are conducted nationwide with key offices located in Chevy Chase, Maryland, Southern California, including Los Angeles, St. Louis, Missouri, Denver, Colorado, Chicago, Illinois, New York, New York, and Midvale, Utah. PacWest also maintains a number of smaller lending offices throughout the country. PacWest operates as two business segments: Community Banking and National Lending. The Community Banking segment is focused on the lending and deposit gathering activities conducted primarily through PacWest's California-based branch offices and PacWest's treasury management function. The National Lending segment comprises PacWest's CapitalSource Division through which PacWest offers a broad range of specialized senior secured commercial loan products to small and middle-market businesses on a nationwide basis.

Square 1 Financial, Inc.
406 Blackwell Street, Suite 240
Durham, North Carolina 27701
Phone: (866) 355-0468

        Square 1 is a financial services company focused primarily on serving entrepreneurs and their investors. Square 1 is headquartered in Durham, North Carolina with thirteen loan production offices located in key innovation hubs across the United States. Through Square 1's banking subsidiary, Square 1 Bank, which was formed by experienced venture bankers, commercial bankers and entrepreneurs, Square 1 offers a full range of banking and financial products focused on the entrepreneurial community and their venture capital and private equity investors. Since inception, Square 1 has operated as a highly-focused venture bank and has provided a broad range of financial services to entrepreneurs, growing entrepreneurial companies and the venture capital and private equity communities. Square 1 provides banking services to its clients, including venture, commercial and international banking services, asset-based lending programs, and SBA and USDA commercial and real estate loan programs. Square 1 also provides investment advisory and asset management services to its clients through Square 1 Asset Management, a subsidiary of Square 1 Bank.

        As of March 31, 2015, Square 1 had consolidated total assets of $3.3 billion, total loans and leases of $1.5 billion, investment securities of $1.7 billion, deposits of $3.0 billion and stockholders' equity of $318.8 million. Square 1 had 270 full-time equivalent employees as of March 31, 2015.

Risk Factors (page 26)

        Before voting at the special meeting, you should carefully consider all of the information contained in or incorporated by reference into this document, including the risk factors set forth in the section entitled "Risk Factors" or described in PacWest's and Square 1's respective Annual Reports on Form 10-K for the year ended on December 31, 2014 and other reports filed with the SEC, which are incorporated by reference into this document. Please see "Where You Can Find More Information" and "Incorporation of Certain Documents by Reference."

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA FOR PACWEST

        The following table summarizes consolidated financial results of PacWest for the periods and at the dates indicated and should be read in conjunction with PacWest's consolidated financial statements and the notes to the consolidated financial statements contained in reports that PacWest has previously filed with the SEC. Historical financial information for PacWest can be found in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2015 and its Annual Report on Form 10-K for the year ended December 31, 2014. Please see the section entitled "Where You Can Find More Information" for instructions on how to obtain the information that has been incorporated by reference. Financial amounts as of and for the three months ended March 31, 2015 and 2014 are unaudited and are not necessarily indicative of the results of operations for the full year or any other interim period, and management of PacWest believes that such amounts reflect all adjustments (consisting only of normal recurring adjustments) necessary for a fair presentation of its results of operations and financial position as of the dates and for the periods indicated. You should not assume the results of operations for past years and for the three months ended March 31, 2015 and 2014 indicate results for any future period.

SELECTED HISTORICAL CONSOLIDATED
FINANCIAL DATA FOR SQUARE 1

        The following table summarizes consolidated financial results of Square 1 for the periods and at the dates indicated and should be read in conjunction with Square 1's consolidated financial statements and the notes to the consolidated financial statements contained in reports that Square 1 has previously filed with the SEC. Historical financial information for Square 1 can be found in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2015 and its Annual Report on Form 10-K for the year ended December 31, 2014. Please see the section entitled "Where You Can Find More Information" for instructions on how to obtain the information that has been incorporated by reference. Financial amounts as of and for the three months ended March 31, 2015 and 2014 are unaudited and are not necessarily indicative of the results of operations for the full year or any other interim period, and management of Square 1 believes that such amounts reflect all adjustments (consisting only of normal recurring adjustments) necessary for a fair presentation of its results of operations and financial position as of the dates and for the periods indicated. You should not assume the results of operations for past years and for the three months ended March 31, 2015 and 2014 indicate results for any future period.