ESR1 and breast cancer: We employed a well-characterized neuroblastoma cell line (SK-N-MC) lacking expression of ERs stably transfected with mock-plasmid (SK-01), estrogen receptor α (SK-ERα) or estrogen receptor β (SK-ERβ).28, 29, 30 These cell lines enable us to study the differential function of ERs in tumor cells under controlled and well-described conditions.21, 28, 30, 31, 32, 33, 34 Additionally, we also employed the patient-derived MCF-7 breast cancer cell line as an ERα-positive breast cancer model endogenously expressing ERα but not ERβ.