MAGE1 encodes a tumor specific antigen MZ2-E that elicited a cytotoxic T lymphocytic response (CTL) in the patient from whom it was derived. In this study, cDNA and genomic probes have been used to localize this gene by Southern analysis of a human-rodent somatic cell hybrid panel. The probes detect a small multigene family, and both MAGEl and several other members of this family are located on the long arm of the human X chromosome. A cosmid with a 12-kb insert including the entire MAGEl gene was biotinylated and used to further localize the gene to Xq28 by in situ hybridization of metaphase spreads. The function of this antigen in normal cells and tumor cells currently remains unclear.