Fig. S1. Fast twitch fibers show pathology in Actb-msKO skeletal muscle. Ten-micron cryosections from Actb-msKO quadriceps stained with antibody to fast myosin heavy chain (left) and slow myosin heavy chain (right). Centrally nucleated fibers (arrows) were observed in fast twitch fibers but not slow twitch fibers (arrowhead). Scale bar: 50 m.

Fig. S2. nNOS localization is not altered in Actb-msKO skeletal muscle. Control, Actb-msKO, and mdx quadriceps sections stained with an antibody to nNOS. Ablation of β_cyto-actin did not alter nNOS localization. Scale bar: 50 m.

Fig. S3. Decreased dystrophin expression in Actb-msKO triceps muscles. (A) Representative western blot of dystrophin from SDS-extracted skeletal muscle. Post-transfer CBB-stained gel shows equivalent loading. The most prominent band represents myosin heavy chain. (B) Quantification of dystrophin levels in Actb-msKO triceps extracts. A 42% decrease in dystrophin expression was detected in Actb-msKO triceps. (C) Ten-micron cryosections from control and Actb-msKO triceps stained with dystrophin (green) and DAPI (blue). Ablation of β_cyto-actin led to decreased dystrophin staining at the sarcolemma. Scale bar: 50 m.

Fig. S4. Histopathology of the diaphragm and extensor digitorium longus muscle in Actb-msKO mice. Representative H&E stained 10 m sections and respective quantification of the proportion of centrally nucleated fibers from control and Actb-msKO diaphragm (A,C), and extensor digitorium longus (B,C) muscles at 12 months of age (n=3 mice per genotype). *P≤0.05. Error bars represent s.e.m. Arrows indicate centrally nucleated fibers. Scale bar: 100 m.