ESR1 and breast cancer: We postulated that changes in fulvestrant-mediated suppression of ERα-FL levels may promote drug resistance and investigated this response in several models selected for fulvestrant resistance (MCF-7 FulvRes, MCF-7 Y537S, two breast cancer PDX models, and T-47D FulvRes)17,24, compared to fulvestrant-sensitive cells (Fig. 1a and Supplementary Fig. 1a, b).