Canine FIX staining and histology of muscle biopsies from dog H48 (3 × 10¹² vg/kg) 2 years after ATVRX delivery of an AAV2-cFIX vector. Left panel, sites of muscle biopsy sampling, AAV-injected target limb: (A) semimembranosus; (B) vastus lateralis; (C) semitendinosus. (A-C) cFIX immunostaining of muscle biopsies collected at the sites indicated. (D) cFIX immunostaining of muscle biopsies from the contralateral, noninjected limb. (E) Hematotoxylin and eosin staining of the muscle biopsy collected from the target leg. Images are taken at 100× magnification.