The FDA’s acceptance of Bluebird’s application, which comes two months after the company submitted it to the agency, marks the final stretch of a long road for the treatment.

Originally, Bluebird had expected to seek U.S. clearance for beti-cel in the first half of 2020, but pushed that target back by six months before a disagreement with the FDA on testing data led to a year-long delay.

In that time, Bluebird won EU approval for beti-cel, struggled mightily to sell it there and opted to withdraw it amid an overall winding down of its business in Europe and a corporate restructuring.

The company’s future is now staked to marketing beti-cel and other gene therapies like it in the U.S. But many question marks still remain, not least of which is whether the FDA will agree and clear beti-cel for use. Bluebird also expects to submit another rare disease gene therapy for U.S. approval by the end of the year, but must first resolve a clinical hold placed by the FDA over safety concerns with treatment. (That therapy was approved in Europe and is also set to be withdrawn from market there due to reimbursement issues.)

A third Bluebird gene therapy, for sickle cell, has proven strongly effective in clinical testing, eliminating the severe pain crises patients with the blood disease often experience. But the company doesn’t expect to submit that treatment for approval until the first quarter of 2023 as it works to validate its commercial manufacturing process and other production criteria.

If approved, beti-cel would give patients with a severe form of beta thalassemia that requires chronic blood transfusions a powerful new therapy. In clinical testing, many patients treated with beti-cel were able to discontinue regular transfusions as the replacement gene delivered by the treatment produces functional hemoglobin protein.