ERBB2 and breast neoplasm: Breast tumors are heterogeneous, and are pathologically classified according to their expression of key proteins by immunohistochemical (IHC) staining (at a minimum 1% [1]): estrogen receptor α (ERα), gene ESR1, tumors are termed ER positive (ER+), progesterone receptor (PR, gene PGR), and human epidermal growth factor receptor 2 (HER2, gene ERBB2).