Two-step mechanism for cell fate decision.

<p>The first step is to decide whether DNA damage is severe enough to activate Casp3 through the mitochondrial p53 pathway. If yes, the apoptotic program is initiated without cell cycle arrest. Otherwise, nuclear p53 transcribes p21, which induces cell cycle arrest to allow DNA repair. Then comes the second step, determining whether the repair is completed. If yes, the cell survives; otherwise, p53 induces expression of pro-apoptotic genes, and apoptosis ensues.