Shares of Bluebird bio Inc. /zigman2/quotes/207450173/composite BLUE -2.42% traded 15.1% into the red in morning trading Thursday, after the gene therapy company pushed back the U.S. submission for approval of its sickle cell disease treatment LentiGlobin to late-2022 from the second half of 2021. The stock was headed toward the lowest close since April 15. Regarding LentiGlobin, the company said late Wednesday the submission delay comes after the Food and Drug Administration requested the use of drug product manufactured from sickle cell disease patient cells in addition to healthy donors, to demonstrate drug product comparability. J.P. Morgan analyst Cory Kasimov kept his rating at overweight but slashed his price target to $76 from $140, saying LentiGlobin in sickle cell disease is "arguably the most important value driver" for Bluebird. "Although there is no change in our view on the clinical profile (it's hard to argue that the data to date has been anything short of impressive), these manufacturing related setbacks are getting old, painful and further call into question the company's ability to execute," Kasimov wrote in a note to clients. BMO Capital's Matthew Luchini downgraded Bluebird to market perform, saying the delay has put the company's execution issues "to the breaking point." The stock has tumbled 43.3% year to date, while the iShares Nasdaq Biotechnology ETF /zigman2/quotes/206189322/composite IBB +0.98% has climbed 16.0% and the S&P 500 /zigman2/quotes/210599714/realtime SPX -2.27% has rallied 8.9%.