Breast cancer can be classified into three types: luminal breast cancers which are usually ER+ and have a relatively good prognosis and response rate to hormonal based therapies, HER2+ cancers which have a poor prognosis if untreated but are initially responsive to herceptin, and basal-like breast cancers which have a poor prognosis and lack expression of HER2, estrogen and progesterone receptors (referred to as “triple-negative”). Here, ESR1 is linked to breast cancer.