There are four molecular subgroups: luminal A (estrogen receptor, ER+; progesterone, PR+; HER2−), luminal B (estrogen receptor, ER+; progesterone, PR+/−; HER2+/−), human epidermal growth factor receptor 2 (HER2)-positive (the HER2 protein helps breast cancer cells grow quickly), and triple-negative breast cancer (TNBC) (estrogen receptor, ER−; progesterone receptor, PR−; HER2−) (Figure 1). The gene discussed is ESR1; the disease is breast cancer.