The term breast cancer expresses not a single disease but it includes four major molecular subtypes (Luminal A, Luminal B, HER2-positive and Basal Like Breast Cancer, BLBC), whose classification is based on the expression of hormone receptors (estrogen receptors (ER), and progesterone receptors (PR)) and Human Epidermal growth factor Receptor 2 (HER2) (Burstein, 2005). The gene discussed is ESR1; the disease is breast cancer.