Breast cancers can be divided into four subtypes: luminal A (Estrogen Receptor (ER)+, Progestogen Receptor (PR)+, HER2− and Ki67 (which is a proliferation marker) <14%), luminal B (ER+, PR+, HER2− and Ki67 ≥14% or ER+, PR+, HER2+), basal-like (ER−, PR− and HER2−), and HER2 positive breast cancer (HER2+, ER− and PR−). Here, ESR1 is linked to breast cancer.