Double Minute 2 Protein (Mdm2, also known as Hdm2) is a RING-finger ubiquitin E3 ligase that acts as a major regulator of the tumor suppressor p53. Mdm2 inhibits p53-mediated cell cycle arrest and Cancer by binding its transcriptional activation domain. The E3 ligase activity is confined to the C-terminal domain of Mdm2 and is responsible for the ubiquitination and subsequent proteasomal degradation of p53. Although the isolated RING domain is capable of p53 ubiquitination, other regions of the protein including a central acidic domain are also crucial for full E3 ligase activity. Mdm2 also regulates its own intracellular levels by auto-ubiquitination, and can be SUMOylated which decreases autoubiquitination activity but increases activity toward p53. Mdm2 also affects the cell cycle and Cancer through interactions with other proteins such as retinoblastoma1 (pRB) and ribosomal protein L5.