The p53 gene is a widely studied anti-oncogene, or tumor suppressor gene. The p53 gene product can act as a negative regulator of cell growth in response to DNA damage. p73 shares a high degree of homology with p53, and appears to have similar growth-inhibiting and apoptosis-promoting functions. However, unlike p53, the expression of p73 is not upregulated in response to DNA damage. p73 can, when overproduced, activate the p53- responsive gene p21. p63 has also been identified based on its similarities with p53. The p63 gene encodes multiple isotypes with variable functions. p63alpha (also designated p51B or KET), p63beta and p63gamma (also designated p51A), as well as corresponding TA* p63 isoforms, contain transactivation domains which have been shown to transactivate p53 reporter genes and induce apoptosis. p63 isoforms lack the transactivation domain and can act as dominant- negative reagents to inhibit transactivation by p53 and p63.