Gallen International Expert Consensus on the primary therapy of early breast cancer (14–16) defined the breast cancer subtypes immunohistochemically as: Luminal A (ER+ and/or PR+, Her2− and low Ki67), luminal B (ER+ and/or PR+, Her2+; ER+ and/or PR+, Her2−, high Ki67), Her2-positive (ER−, PR− and Her2+) and the triple-negative type (ER−, PR− and Her2−). Here, ESR1 is linked to breast cancer.