ESR1 and breast cancer: In breast cancer cells, studies havebeen reported showing thatgenetically modified T47D and MCF7 breast cancer cells containingmutations in estrogen receptor 1 (ESR1) (Y537S and D538G) showed estrogen-independentgrowth and resistance to fulvestrant, raloxifene, and 4-hydroxytamoxifen(4-OHT) in vitro.