To date, BC is routinely distinguished in three broad subtypes based on the expression of diagnostic biomarkers: luminal (estrogen (ER) and progesterone (PgR) receptors positive, human epidermal growth factor receptor 2 (HER2) positive or negative), HER2-enriched (ER and PR negative, HER2 positive) and triple-negative (ER, PR, and HER2 negative) BC. The gene discussed is ESR1; the disease is breast cancer.