Approximately two-thirds of the population had triple-negative breast cancer, defined as ER less than 1%, PR less than 1%, and ERBB2 nonamplified (immunohistochemistry subtype 0, 1, or 2 with negative in situ hybridization), whereas one-third had hormone receptor–positive breast cancer and 4 individuals had ERBB2-amplified MpBC. This evidence concerns the gene ESR1 and breast cancer.