Summary of the mechanism of action of the tumor suppressor protein, p53. After DNA damage, the tumor suppressor protein, p53, will be upregulated causing cell cycle arrest and enhancing DNA repair. However, in cases of irreversible DNA damage, p53 has been shown to transcriptionally repress the antiapoptotic gene Bcl-2, while it upregulates the pro-apoptotic proteins Bax and Fas. This in turn, promotes apoptosis. During apoptosis loss of the integrity of the mitochondrial membrane is followed by release of cytochrome c into the cytosol, this in turn leads to activation of caspase cleavage. Bax has been shown to contain p53-binding sites in its promoter site and is upregulated in response to DNA damage and increased p53 .