Breast cancers are classified as five intrinsic subtypes based on their gene expression profiles revealed by microarray: luminal-like subtypes A and B (expression of hormone receptors and luminal cytokeratins 8 and 18), basal-like (also called triple-negative breast cancer (TNBC), typically with no expression of estrogen receptors (ER), progestin receptors, and HER2), HER2-positive (HER2+), and normal-like [28,29]. Here, ESR1 is linked to breast cancer.