Depletion of MyoD in growth-stimulated quiescent myoblasts is associated with a reduction in the number of cells entering S phase. (A) C2C12 myoblasts were transduced with lentiviruses (shMyoD-66 or shRNA control), and after 48 h, they were brought to quiescence, at which time serum (GM) was then added for either 16 or 36 h. (B) To monitor DNA synthesis, BrdU was added to the media for 30 min at each of the respective times, and then immunostained, accordingly, for the detection of BrdU incorporation. The bottom panel is the quantitation of the results, and the bars in the histogram represent the mean percentages of cells positive for BrdU per field of view. Error bars represent SEM. (*, P < 0.001). Bar, 20 µm.