ERBB2 and breast cancer: MCF7 is a luminal A subtype of breast cancer cell line, which is characterized by the presence of two hormone receptors, estrogen receptor 1 (ER1 or ERα, encoded by gene ESR1) and progesterone receptor (PR, encoded by gene PGR), and absence of human epidermal growth factor receptor 2 (HER2, encoded by gene ERBB2).