Gilead Sciences has signed a definitive agreement to acquire YM BioSciences in transaction valued at approximately $510 million in cash. The transaction is expected to close in 1Q13.   YM’s lead drug candidate, CYT387, is an oral selective inhibitor of the Janus kinase (JAK) family, which has been implicated in a number of disorders including myeloproliferative diseases, inflammatory disorders and certain cancers. YM reported positive results from a Phase I/II trial of CYT387 in patients with my...