Four-week-old mdx;Tak1^fl/fl and mdx;Tak1^mKO mice were treated with tamoxifen and analyzed at the age of 8 weeks. Transverse sections of TA muscle were immunostained for Pax7 and Laminin. Nuclei was counterstained with DAPI. (A and B) Representative photomicrographs and quantitative analysis showing frequency of Pax7⁺ cells in TA muscle of 8-week-old littermate mdx;Tak1^fl/fl and mdx;Tak1^mKO mice. n = 4 per group. Data represented as mean ± SEM. *P ≤ 0.05, values significantly different mdx;Tak1^fl/fl mice by Student t test. (C–E) Transverse sections of GA muscle were immunostained for eMyHC and Laminin. Nuclei were counterstained with DAPI. (C) Representative photomicrographs and quantitative analysis of (D) average myofiber cross-section area of eMyHC⁺ myofibers and (E) percentage of eMyHC⁺ myofibers in 8-week-old mdx;Tak1^fl/fl and mdx;Tak1^mKO mice. n = 4 per group. (F and G) Western blot and densitometry analysis showing protein levels of eMyHC, MyoD, and unrelated protein GAPDH. n = 6. Data represented as mean ± SEM. *P ≤ 0.05, values significantly different from mdx;Tak1^fl/fl mice by Student t test. (H–J) Following TAK1 inactivation in adult Tak1^fl/fl and Tak1^mKO mice, TA muscle was injected 100 μL of 10 μM cardiotoxin solution. (H) Representative photomicrographs of H&E-stained TA muscle sections and quantification of (I) average myofiber cross-section area (CSA) and (J) percentage of centronucleated fibers (CNFs). n = 4–5 per group. Data represented as mean ± SEM. Western blots for F, Figure 5E, and Figure 6E were performed contemporaneously. Black lines on the immunoblots indicate that intervening lanes have been spliced out.