In previous analyses, dual expression of melanocytic and epithelial molecular markers have been described as indicators of lineage infidelity for breast cancer cells that lose their epithelial identity. Here we demonstrated that this is a much more frequent phenomenon in human breast carcinomas, usually affecting only a part of the tumor. Accordingly we detected, in 18 out of 100 breast carcinomas, immunohistochemically focally positive cells for the melanocytic marker Melan A. The presence and extent of Melan A expression was statistically significantly associated with a reduction in tumor cell differentiation, but not tumor type, size, lymph node metastasis, hormone receptor status or Her-2-neu expression. Microarrays of a further 159 breast cancers showed, in several samples, variably low expression levels of Melan A (and other melanocytic markers) that are consistent with focal expression in many tumors. One case strongly overexpressed Melan A. The transition from an epithelial to a melanocytic phenotype (lineage infidelity) appears to occur much more frequently than previously assumed and occurs in restricted areas of breast cancer during tumor progression, a possible association with a reduction in tumor cell differentiation.