The finding that GPER and ER are independently expressed in breast cancer suggests that ER-only-targeted endocrine treatments, including those under development, adequately treat only 19% of breast cancer patients (ER+/GPER−, red-shaded area), while 43% of all patients (ER+/GPER+, blue-shaded area) are treated partially. Here, ESR1 is linked to breast cancer.