The MDM2 oncogene was first cloned as an amplified gene on a murine double-minute chromosome in the 3T3DM cell line, a spontaneously transformed derivative of BALB/c 3T3 cells. The MDM2 oncogene has now been shown to be amplified or overexpressed in many human cancers. It also has been suggested that MDM2 levels are associated with poor prognosis of several human cancers. The most exciting finding is the MDM2-p53 autoregulatory feedback loop that regulates the function of the p53 tumor suppressor gene. The MDM2 gene is a target for direct transcriptional activation by p53, and the MDM2 protein is a negative regulator of p53. The MDM2 oncoprotein binds to the p53 protein, inhibiting p53 functions as a transcription factor and inducing p53 degradation. The p53 tumor suppressor has an important role in cancer therapy, with p53-mediated cell growth arrest and/or apoptosis being major mechanisms of action for many clinically used cancer chemotherapeutic agents and radiation therapy. Therefore, the MDM2-p53 interaction may be a target for cancer therapy. In addition, the negative regulation of p53 by MDM2 may limit the magnitude of p53 activation by DNA damaging agents, thereby limiting their therapeutic effectiveness. If the MDM2 feed-back inhibition of p53 is interrupted, a significant increase in functional p53 levels will increase p53-mediated therapeutic effectiveness. Several approaches have now been tested using this strategy, including polypeptides targeted to MDM2-p53 binding domain and antisense oligonucleotides that specifically inhibit MDM2 expression. In addition to the interaction with p53, the MDM2 protein has been found to have interactions with other cellular proteins such as pRb and E2F-1. Although the exact function and significance of these interactions are not fully understood, the p53-independent functions of MDM2 may have a role in cancer etiology and progression, indicating that the MDM2 oncogene is a potential molecular target for cancer therapy.