Regeneration of TA muscle and myoblast differentiation comparing Per1^−/−, Per2^−/−, and Per1^−/−:Per2^−/− mice. (A) Average of the minimal Feret’s diameters of myofibers with centrally located nuclei on day 14. TA muscle was injured with barium chloride at ZT14 on day 0 and harvested 14 d later. n = 8 mice with 4 males and 4 females in each group in A and B. (B) Average of the minimal Feret’s diameters of myofibers in uninjured mice. (C–F) EdU uptake and immunofluorescence staining of undifferentiated (C) and differentiating primary myoblasts on day 1 (D), day 2 (E), and day 3 (F) with antibodies against MHC and MyoD. DNA was counterstained with DAPI. Bars, 100 µm. *, P < 0.05 and ***, P < 0.001 with Student’s t test compared with WT mice. Data are presented as mean + SEM.