** Shares of gene therapy developer bluebird bio BLUE.O fall 1.8% to 36 cents; set to snap a three-day winning streak
** The U.S. FDA says it is weighing the need of regulatory action on BLUE's gene therapy, Skysona, used to treat a rare neurological disorder called cerebral adrenoleukodystrophy
** Additional cases of serious blood cancers reported in patients treated with Skysona, FDA says; therapy's prescribing information already warns of those risks
** FDA advises exploring alternative therapies before using Skysona, considering the risk of blood cancers
** Including session's gains, stock has fallen 73.6% YTD and has been trading below the $1-mark since mid-August
(Reporting by Kamal Choudhury in Bengaluru)
((Kamal.Choudhury@thomsonreuters.com;))