In vitro, MDM2 enhances the proliferation of human breast cancer cells and antagonizes apoptosis [38–43]; in vivo, Mdm2 transgene expression initiates mammary gland tumorigenesis in murine models [44]; and in breast cancer patients, MDM2 protein overexpression and MDM2 gene amplification are associated with decreased overall and/or disease-free survival [45–48]. Here, MDM2 is linked to breast cancer.