Overexpression of hemagglutinin (HA)-K63R affects muscle development. (A) Monitoring of litter sizes showed that the higher-expressing 654 line gave rise to 50% fewer pups than the 3405 line or wild-type FVB/N (* P<0.05 for the 3405 or FVB/N t-test comparison). (B, C) Intercostal muscles of embryos at 13.5 days post-conception (dpc) stained for myosin heavy chain (MF20). (D, E) Forelimb muscles of the same embryos stained with MF20. Transgenic animals had smaller and more disorganized muscle fibers (arrowheads). (F, G) Immunohistochemistry for myogenin in 11.5 dpc embryos showing smaller myogenic compartments along the rostral-caudal axis in the 654 transgenic line. (H, I) Immunohistochemistry for MF20 as above in 11.5 dpc embryos.