ESR1 and breast adenocarcinoma: Response to 17β‐estradiol was assessed in ER+ breast cancer models with resistance to estrogen deprivation: WHIM16 patient‐derived xenografts, C7‐2‐HI and C4‐HI murine mammary adenocarcinomas, and long‐term estrogen‐deprived MCF‐7 cells.