p53 (also known as TP53) is a tumor suppressor protein critical for cell cycle control and apoptosis. Its family members also include p63 and p73. p53 is composed of four homologous subunits and is primarily localized in the nucleus, where it monitors DNA damage and prevents the proliferation of mutated cells. p53 participates in multiple signaling pathways by activating the expression of DNA repair proteins, cell cycle inhibitors, and apoptosis-promoting genes. Mutations in the p53 gene are a common molecular event in many human cancers.