Gilead Sciences Inc. (Nasdaq: GILD) announced that it would acquire YM BioSciences Inc. (NYSE: YMI), a developer of a treatment for progressive, chronic bone marrow disorder, in an all-cash deal valued at approximately $500 million. Shares of YM BioSciences leaped $1.25 to $2.88 while Gilead stock edged up 26 cents to $76.60.