Breast cancers can be subdivided according to their expression status for estrogen receptor, progesterone receptor, and Her2 into luminal A (ER/PR+, Her2−), luminal B (ER/PR+, Her2+), Her2-positive (ER/PR−, Her2+), and basal-like/triple negative (triple-negative breast cancer, ER/PR−, Her2−) (Perou et al., 2000). Here, ESR1 is linked to breast cancer.