ESR1 and breast cancer: Of these 104 patients, 56 (53.8%) were HR+/HER2- (defined as ER+ and/or PgR+, and HER2-), 26 (25.0%) were HER2+ (defined as HER2+, irrespective of ER or PgR status), and 22 (21.2%) were triple-negative breast cancers (TNBC, defined as ER-, PgR-, and HER2-) at the time of primary breast cancer diagnosis.