Breast cancer is a heterogeneous disease and currently is split into three subtypes clinically based on immunohistochemical analysis: ERα +ve (oestrogen receptor α-positive), HER2 +ve (human epidermal growth factor receptor 2-positive), and triple negative (cancers that lack expression of ERα, progesterone receptor (PR), and HER2). The gene discussed is ESR1; the disease is breast carcinoma.