Satellite cells are myogenic precursors responsible for skeletal muscle regeneration. Satellite cells are absent in the Pax-7-/- mouse, suggesting that this transcription factor is crucial for satellite cell specification [Seale, P., Sabourin, L.A., Girgis-Gabardo, A., Mansouri, A., Gruss, P., Rudnicki, M.A., 2000. Pax7 is required for the specification of myogenic satellite cells. Cell 102, 777-786]. Analysis of Pax-7 expression in activated satellite cells unexpectedly revealed substantial heterogeneity within individual clones. Further analyses show that Pax-7 and myogenin expression are mutually exclusive during differentiation, where Pax-7 appears to be up-regulated in cells that escape differentiation and exit the cell cycle, suggesting a regulatory relationship between these two transcription factors. Indeed, overexpression of Pax-7 down-regulates MyoD, prevents myogenin induction, and blocks MyoD-induced myogenic conversion of 10T1/2 cells. Overexpression of Pax-7 also promotes cell cycle exit even in proliferation conditions. Together, these results suggest that Pax-7 may play a crucial role in allowing activated satellite cells to reacquire a quiescent, undifferentiated state. These data support the concept that satellite cell self-renewal may be a primary mechanism for replenishment of the satellite cell compartment during skeletal muscle regeneration.