The p53 transcription factor prevents tumor development through induction of cell cycle arrest and cell death by apoptosis. As many as several hundred genes or more are regulated by p53. Around half of all human tumors carry p53 mutation, mostly point mutations that abrogate p53's specific DNA binding and transactivation activity. p53 mutation is associated with poor therapeutic response and prognosis. Tumors that carry wild type p53 often have other alterations in the p53 pathway that ablate the p53 response. Several strategies have been designed to restore p53 function in human tumors, including p53 gene therapy, reactivation of mutant p53, and activation of wild type p53 by inhibition of the p53 antagonist MDM2. In all cases, the aim is to eliminate the tumor through induction of massive apoptosis.