Little is known about the expression of cell cycle regulatory genes upon terminal differentiation of skeletal muscle cells. In this report, we demonstrate that the expressions of cyclin A, cyclin D1 and p34cdc2 are downregulated upon C2C12 myocytes differentiation and are not inducible in differentiated myotubes. SV40 large T antigen can induce cell cycle entry of myotubes through its induction of these genes' expressions and pRB phosphorylation as well as its suppression of Rb expression. These results provide the first direct evidence that the irreversible downregulation of cyclins and cyclin-dependent kinases is one mechanism for the permanent cell cycle withdrawal of myotubes.