Generally, breast cancer can be classified into four different molecular subtypes, according to the expression/expression levels of estrogen receptor (ER), progesterone receptor (PR), Erb-B2 receptor tyrosine kinase 2 [ERBB2/human epidermal growth factor receptor 2 (HER2)], and Ki-67 (a cell proliferation marker) in cancer cells. The gene discussed is ESR1; the disease is cancer.