AAV vector-mediated gene therapy shows promise as a treatment for hemophilia B. Recent clinical trials using AAV5 vectors with liver-specific promoters demonstrated significantly improved patient symptoms and decreased need for factor IX injections by achieving clinically meaningful increases in FIX expression. While expression was sustained for up to 3 years in trials, limitations include the potential for immune responses, insufficient transgene expression, and uncertainty around long-term safety and cost-effectiveness. Overall, AAV5 vectors appear to be the most clinically successful approach thus far.