Etranacogene dezaparvovec (Hemgenix) is the only commercially available gene therapy for the treatment of hemophilia B.[1] CSL Behring has announced that the drug is temporarily unavailable in the United States due to stockouts.[1] The manufacturer warns against treatment delays and has addressed a letter to the hemophilia B patient community.[1] Hemgenix is a single-use infusion that expresses human coagulation factor IX in the liver using an AAV5 vector.[1][3] The drug is approved in the USA, EU and other countries; so far, over 75 patients in eight countries have received it in the real world.[5] Five-year data from the phase 3 HOPE-B study confirm long-term efficacy and safety, including a 64% reduction in bleeding.[5] The most common adverse effects included elevation of alanine aminotransferase, in which 16.7% of participants received corticosteroids.[5]