Adenine inhibits cell cycle regulators but not caspase effectors. Cells were treated with 1.0, 2.0 and 4.0 mM adenine for 24 h, and then lysed for determination of protein expression levels using western blot analysis. GAPDH was used as an internal control. The apparent molecular weights of the detected proteins are indicated. PARP, poly-ADP-ribose polymerase; CDK, cyclin-dependent kinase; Cdc25A, cell division cycle 25A; Wee1, Wee1-like protein kinase; C, control.