MDM2 (Murine Double Minute 2), an E3 ubiquitin ligase and a negative regulator of p53, is a 56 kDa oncoprotein which is ubiquitinated and phosphorylated. MDM2 degradation is controlled by self-ubiquitination, phosphorylation, and potentially through ubiquitination by other not yet identified E3 ligases. DNA damage and cellular stress trigger MDM2 degradation, releasing p53 from MDM2 mediated negative regulation. Deletion of MDM2 in mouse models is lethal in a p53 dependent manner, and overexpression of MDM2 is seen in many cancers with non-mutated p53, leading to the conclusion that MDM2 is oncogenic by way of p53 inactivation. Due to the important role p53 tumor suppression plays in many different forms of cancer, there has been extensive research on the interactions between MDM2 and p53 and considerable interest in identifying drugs capable of modulating the MDM2 - p53 interaction. This kit provides the assay-specific components for the quantitative determination of MDM2 in human whole cell lysates.