Gene expression profiling by DNA microarray have identified the inherent classification of BC into five main molecular subtypes: Luminal A (estrogen receptor (ER) +/progesterone receptors (PR) +/epidermal growth factor receptor 2 (HER2; ERBB2) –) are commonly of low score; luminal B (ER+/PR–/+/HER2+/–) are normally of higher score with more proliferation rate; HER2-enriched subtype (ER–/PR–/HER2+); triple-negative breast cancer (TNBC; ER–/PR–/HER2–); and carcinomas that are analogous to normal breast tissue and is associated with good prognosis (2–4). Here, ESR1 is linked to breast cancer.