The health regulator sought certain additional data to satisfy requirements pertaining to chemistry, manufacturing, and controls before the treatment, called pz-cel, can be approved, in its so-called "complete response letter". The drug developer is seeking approval for the treatment in patients with recessive dystrophic epidermolysis bullosa, where the skin becomes fragile and blisters can appear through rubbing or exposure to heat. FDA in its response letter indicated that the proposed timing of the data submission would not allow sufficient time to complete its review by May 25, the set deadline to decide on the application.