p53 plays an important role in regulating the cell cycle and can function as a tumor suppressor. It was first discovered in 1979 as a 53 kDa protein encoded by the TP53 gene. p53 has several domains that allow it to activate DNA repair when damage is detected, arrest the cell cycle to allow time for repair, or initiate apoptosis if damage is irreparable. Mutations in p53 are common in many cancer types and can be caused by chemical carcinogens and radiation exposure.