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MDM2, also known as HDM2, is a protein of the E3 ubiquitin-ligase family. MDM2 is recognized as a proto-oncogene and is over-expressed in a wide range of human malignancies including soft tissue carcinomas and breast cancer. MDM2 can bind to the p53 tumor suppressor protein with high affinity, and negatively regulates p53 by mediating its ubiquitination and proteosomal degradation (PMID: 17000718). MDM2 has some isoforms with calculated molecular mass of 25-55kD, and can be detected as 90-97kD. In addition to full length MDM2, two lower bands (85kD and 75kD) are often observed which are likely to be produced by translation initiation at methionine codons 50 and 60. Another isoform of MDM2 is often observed as 57-60kD in cell lines which is indeed a caspase cleavage product of MDM2 at residue 361 (PMID: 19565011, 9840926, 7689721).