The Mdm2 proto-oncogene is amplified and over-expressed in a variety of tumors. One of the major functions of Mdm2 described to date is its ability to modulate the levels and activity of the tumor suppressor protein p53. Mdm2 binds to the N-terminus of p53 and, through its action as an E3 ubiquitin ligase, targets p53 for rapid proteasomal degradation.