Long‐term estrogen‐deprived and FR cells, as well as WHIM16 patient‐derived xenografts and C7‐2‐HI murine mammary adenocarcinoma allografts that regress upon E2 treatment, exhibit genomic amplification of ESR1. In contrast, ESR1 is not amplified in C4‐HI tumors, which are only partially responsive to E2 therapy [Figs 1 and 2H,I, Fig. S1, and ref. (Puenpa et al., 2013)]. Here, ESR1 is linked to breast adenocarcinoma.