ERBB2 and breast cancer: Breast cancer is a heterogeneous disease that is clinically subtyped based on the levels of expression of three receptors: tumors expressing estrogen and/or progesterone receptors are classified as HR+, tumors overexpressing and/or amplified for the HER2 receptor tyrosine kinase are classified as HER2 positive, and those lacking expression of these three genes are classified as triple-negative breast cancer (TNBC).