Etranacogene dezaparvovec is a gene therapy for the treatment of hemophilia B that uses an adeno-associated virus vector to deliver Padua factor IX (FIX) variant to the liver. In an open-label phase 2b study in three adult patients with severe or moderate hemophilia B, a single intravenous dose increased FIX activity from ≤2% to a mean of 30.6 IU/dL at 6 weeks, and this activity remained stable at approximately 45.7 IU/dL at 5 years of follow-up[1]. No serious safety issues, such as elevation of liver enzymes, development of FIX inhibitors, thrombotic complications or need for steroid treatment, were reported over a five-year period [1]. In a phase 3 study of 54 men with hemophilia B, the annual bleeding rate was reduced from 4.19 to 1.51, confirming the superiority of gene therapy over standard FIX prophylaxis[2]. After therapy, FIX prophylaxis could be completely stopped in 96% of patients and the number of infusions was significantly reduced[3]. The most common side effects were transient and included increased liver enzymes, headache, and flu-like symptoms[3]. One case of hepatocellular carcinoma was not associated with gene therapy[3]. Etranacogene dezaparvovec represents a significant advance in the treatment of hemophilia B with proven long-term efficacy and a favorable safety profile[1][2][3].