F9 and hemophilia B: Recombinant adeno-associated virus (AAV) gene delivery systems have been successfully deployed in gene therapies that are marketed or in development to treat hemophilia B. Etranacogene dezaparvovec is an AAV serotype 5 (AAV5)-based gene therapy with a codon-optimized gene expression cassette encoding the naturally occurring human factor IX Padua (R338L) variant.1