In-depth molecular assessment of breast tumors has corroborated the established quartet of breast cancer subtypes previously characterized via immunohistochemistry (Box 1): luminal A and B tumors, characterized by the expression of estrogen and progesterone receptors and by differing proliferation rates; HER2 (ERBB2)-amplified tumors; and the triple-negative entity. This evidence concerns the gene ERBB2 and breast cancer.