[Mdm2, p53 and the cell cycle: when well enough is best left alone].

The mdm2 cellular protooncogene is involved in many human tumors where it has been shown to be overexpressed, including sarcomas, osteosarcomas, gliomas and others. The Mdm2 protein is believed to be oncogenic by binding and inactivating the p53 and Rb tumor suppressor gene products and by activating the E2F-1/DP-1 transcription factors, thus promoting the G1 to S phase transition. The mdm2 gene is activated transcriptionally by p53, thus forming an autoregulatory negative feedback loop. This feedback loop is important in normal cells and when cells are exposed to various genotoxic agents. By activating its own negative regulator, p53 would signal the cells to resume proliferation after a p53-mediated G1 arrest in response to DNA damage. The review aims to detail the functions of Mdm2 in normal and tumor cells. We also discuss several recent data suggesting that Mdm2 may exhibit activities unrelated to its well known function as a negative regulator of p53 activities.