ESR1 and breast cancer: Twenty-one studies (84%) provided information on hormone receptor (HR) status among patients with HER2+ BC, reporting a range of 24.2% [33] to 71.0% [26] of patients with HR+ status (i.e., estrogen receptor positive and/or progesterone receptor positive), while four studies (16%) did not report on HR status [16, 30, 34, 35].