Syncytial sheets of high-grade tumor cells are separated by cords of lymphocytes and plasma cells. The immunohistochemical profile of medullary carcinoma does not differ greatly from conventional invasive ductal carcinonma. They are usually positive for CK7, S-100 protein, and P53. Cytokeratin 20 is generally negative. ER, PR, and HER2 are negative (triple negative phenotype).