Expression of hemagglutinin (HA)-K63R enhances muscle regeneration. Muscle repair after cardiotoxin-induced injury was monitored at day 7 post-injection in (A) wild-type and (B) line 654 transgenic animals using hematoxylin and eosin staining. More damage was consistently seen in the wild-type animals. Insets show representative regenerates at day 14 post-injury, (C) The extent of damage was quantified by measuring the proportion of fibers with specific cross-sectional area (CSA) in the damaged area. Approximately 500 fibers were counted from 3 different animals for all genotypes. A shift towards an increased proportion of larger fibers was seen at day 7 for the transgenic groups, suggesting enhanced regeneration. No differences were seen at day 14. (D-G) Regenerating sections at day 3 from wild-type and transgenic mice were stained for MyoD and Pax7 to identify activated satellite cells (arrowheads). (H) Activated satellite cells were enumerated from at least three different animals at days 3 and 7 post-injury, and expressed as the proportion of total nuclei in the field. No differences were seen between the transgenic and wild-type animals.