Molecular subtypes of breast cancer include human epidermal growth factor receptor-2 (HER-2), triple-negative breast cancer (TNBC), estrogen (ER)/progesterone receptor (PR)-positive or -negative molecular subtypes, and luminal A and luminal B [4] ER/PR-positive types of breast cancer are almost 70% of the cases usually reported. The gene discussed is ESR1; the disease is breast cancer.