From the molecular level and according to the expression and/or lack expression of hormone receptors (HR); estrogen receptors (ER) and progesterone receptors (PR) and HER-2 on breast cancerous cells, breast cancer is categorized into four subtypes: Luminal A and B characterized by expression of hormonal receptors with or without HER-2, basal-like cancerous cells where the three hormonal receptors are deficient, so they are commonly known as triple negative breast cancer (TNBC), and HER-2 enriched cancer that is enriched with HER-2 only [6,8]. Here, ESR1 is linked to breast cancer.