The majority (68–73%) of women with breast cancer present with hormone receptor-positive (HR+; estrogen receptor-positive [ER+] and/or progesterone receptor-positive [PgR+]) and human epidermal growth factor receptor 2-negative (HER2−) disease, with 14–15% presenting with HER2+ disease and 10–12% with triple-negative disease2,3. Here, ESR1 is linked to breast cancer.