p53 is a sequence-specific DNA binding protein that regulates transcription. p53 is maintained at low levels in normal cells through mouse double minute 2 (MDM2) mediated degradation, however it is stabilised by phosphorylation and thus accumulates in response to cellular stresses such as DNA damage and replication stress. When p53 accumulates it promotes cell cycle arrest and apoptosis, thereby reducing the risk of propagating mutations. p53 mediates the DNA damage-induced G1 phase checkpoint, partly via its ability to transcriptionally activate the p21 cyclin-dependent kinase inhibitor gene. Activation of p21 triggers cell cycle arrest to allow for DNA repair prior to further cell division. p53 therefore acts as a tumour suppressor and inactivating mutations in the human p53 gene TP53 are common in many cancers