Hereditary cutaneous malignant melanoma (HCMM) with or without atypical moles has been described in several large kindreds worldwide. Despite numerous studies of these kindreds, the gene or genes responsible for this disorder has not yet been identified. Cytogenetic and molecular studies of melanoma tumor tissue and cell lines suggest that a gene involved in the pathogenesis of malignant melanoma lies on chromosome 9p. We describe a woman with atypical moles and multiple primary melanomas, who lacks a family history of HCMM, and, has a de novo constitutional unbalanced reciprocal translocation involving the short arms of chromosomes 5 and 9 with a cytogenetically visible deletion of 5p or 9p: [46,XX,-5,-9, +der(5)t(5;9) (p13.3 or 14.2;p13.3 or 21.2), +der(9)t(5;9)]. This finding supports the hypothesis that a gene predisposing to HCMM lies on the short arm of chromosome 9.