Tumor suppressor protein p53, a nuclear transcription factor, plays an essential role in the regulation of cell cycle and is frequently mutated or inactivated in many cancers. Numerous post-translational modifications modulate p53 activity including ubiquitination, phosphorylation, acetylation and methylation. The stability of p53 is regulated via the ubiquitin-proteasome pathway (UPP). MDM2 is an oncogenic ubiquitin E3 ligase that ubiquitinates p53, inhibits its transcriptional activity and promotes its degradation. Other E3 ligases that promote the proteasome-mediated degradation of p53 include Pirh2, COP1 and p300. USP7 (HAUSP) stabilizes p53 by deubiquitination and induces p53-dependent cell growth repression and apoptosis. Additional factors such as p14^ARF and MdmX also modulate p53 function via the UPP. This recombinant protein is N-terminally tagged and produced in E.coli. Accession # NP_000537.