Cytogenetic studies of a breast adenolipoma (hamartoma) of a 58-year-old patient revealed a karyotype 46,XX,add(4)(?),add(6)(q?),der(7)t(7;12)(q11.1 or q11.2;q11 or q12),der(12). To our knowledge, this is the second report of an aberration involving 12q12-15 in a hamartoma of the breast. By FISH studies, we found this chromosome 12 translocation breakpoint to be mapping within the MAR (Multiple Aberration Region). MAR is known to be a major cluster region of chromosome 12 breakpoints of benign solid tumors such as uterine leiomyoma, lipoma, and pleomorphic salivary gland adenomas, therefore raising the possibility that the same gene is involved in hamartoma of the breast as in these three benign solid tumors.