Immunofluorescent staining for hF.IX in AAV-injected mouse muscle. Animals were injected in the hindlimbs with 4 × 10¹² vg/kg AAV-1 or AAV-2, divided equally among 4 injection sites. Twelve weeks later mice were killed and injected muscle was resected for immunofluorescent staining. Positive cells were scored by observers blinded to vector type. Representative sections from the quadriceps are shown for AAV-1– and AAV-2–injected muscle; average percent of positive cells was 54% for AAV-1, 22% for AAV-2.