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Mouse double minute 2 homolog （MDM2） also known as E3 ubiquitin-protein ligase Mdm2 is a protein that in humans is encoded by the MDM2 gene. Mdm2 is an important negative regulator of the p53 tumor suppressor. Mdm2 protein functions both as an E3 ubiquitin ligase that recognizes the N-terminal trans-activation domain （TAD） of the p53 tumor suppressor and as an inhibitor of p53 transcriptional activation. Mdm2 overexpression was shown to inhibit DNA double-strand break repair mediated through a novel, direct interaction between Mdm2 and Nbs1 and independent of p53. Several human tumor types have been shown to have increased levels of Mdm2, including soft tissue sarcomas and osteosarcomas as well as breast tumors.