DNA synthesis in differentiating skeletal muscle cells: initiation by ultraviolet light.

As skeletal muscle cells differentiate, they fail to initiate DNA synthesis. This rigid regulation, which persists even after cells are fully developed, does not extend to "repair" DNA synthesis, in that ultraviolet light initiates DNA synthesis in 99 percent of the muscle nuclei exposed. The rate of "repair" DNA synthesis in these nuclei, however, drops over 50 percent at the time of cell differentiation.