Trial for Hemophilia B

This study will evaluate the efficacy and safety of PF-06838435 (a gene therapy drug) in adult male participants with moderately severe to severe hemophilia B (participants that have a Factor IX circulating activity of 2% or less). The gene therapy is designed to introduce genetic material into cells to compensate for missing or non-functioning Factor IX. Eligible study participants will have completed a minimum 6 months of routine Factor IX prophylaxis therapy during the lead in study (C0371004). Participants will be dosed once (intravenously) and will be evaluated over the course of 6 years. The main objectives of the study are to compare the annualized bleeding rate [ABR] of the gene therapy to routine prophylaxis from the lead-in study and to evaluate the impact that it may have on participant's Factor IX circulating activity [FIX:C].