Like many E3 ligases, MDM2 undergoes autoubiquitylation. Although it was originally thought that MDM2 autoubiquitylation promotes autodegradation, it has recently been shown that this may not be the case, but rather, autoubiquitylation may actually activate the E3 ligase activity of MDM2, and promote p53 targeting. This is proposed to occur through recruitment of E3 conjugating enzyme to the poly-ubiquitin chain on MDM2. Mdm2 that has been conjugated to polyubiqui-tin chains, but not to single ubiquitins, exhibits substantially enhanced activity to polyubiquitinate p53. Echanistically, auto-ubiquitination of Mdm2 facilitates the recruitment of the E2 ubiquitin-conjugating enzyme. Mutations that diminish the noncovalent interactions render auto-ubiquitination unable to stimulate Mdm2 substrate E3 activity. The specificity and efficiency of ubiquitination are largely determined by the E3, which binds to both an E2 thio-esterified with ubiquitin (E2Ub) and a substrate protein, and stimulates the transfer of ubiquitin from E2Ub to the sub-strate.