The p53 tumor suppressor protein is a multi-functional transcription factor that regulates cellular processes affecting proliferation, cell cycle checkpoints, and apoptosis. Binding of MDM2 regulates p53 activity by targeting p53 for degradation and inhibits p53- induced cell cycle arrest and apoptosis. In response to genotoxic stress signals or DNA damage, p53 is phosphorylated and accumulates in the nucleus. p53 can cause cell cycle arrest to allow for DNA damage repair or can cause the cell to undergo apoptosis if the damage cannot be repaired. In addition, p53 has been shown to trigger apoptotic pathways independent of its activity as a transcription factor.