F9 and hemophilia B: The first demonstration of highly efficient, nuclease-mediated gene editing in vivo used AAV vectors to deliver ZFNs and a factor IX cDNA, without a promoter, to the liver of a mouse model of hemophilia B.70 Cleavage of the first intron of a mutated human factor IX gene by ZFNs catalyzed the efficient integration of the factor IX cDNA into the locus, leading to correction of the hemophilic phenotype.