Breast cancer subtype was determined in metastases first-hand and primary tumors second-hand, with 94 patients (69%) having estrogen receptor-positive (ER +) and human epidermal growth factor receptor 2 negative (HER2-) tumors, 15 patients (11%) had HER2 + (ER ±) tumors, and 24 patients (18%) had triple-negative breast cancer (TNBC), with the subtype missing for two patients. This evidence concerns the gene ESR1 and breast cancer.