The MDM2 gene is a gene whose product binds to p53 and regulates its functions. The amplification of the MDM2 gene has been found in one third of human sarcomas, and a differential expression of MDM2 gene in relation with oestrogen receptor status was recently found in human breast cancer cell lines. We analysed 60 breast cancers for MDM2 gene amplification by Southern blot. This event was observed in 1 case with high levels of oestrogen receptor (ER). Thus, MDM2 gene amplification seems to be a rare event in breast cancer. Further studies are needed to define precisely the relationship between MDM2 amplification and ER status.