Abstract
We present the case of a 77 year old patient with a primary breast carcinosarcoma composed mostly of an osteogenic sarcoma of fibroblastic type in which only immunohistochemical analysis disclosed the presence of a minor malignant epithelial component. The malignant mesenchymal component derives from dedifferentiation of myoepithelial cells since myoepithelial markers are positive. Also, like the majority of the other metaplastic carcinomas in the breast, carcinosarcoma is a basal type of tumor that will not respond to endocrine drugs or Her2/neu therapy.