ESR1 and breast cancer: About 75% of patients with a BRCA1 mutation are disposed to triple-negative breast cancer (TNBC; estrogen-receptor (ER)-negative, progesterone-receptor (PR)-negative, and human epidermal growth factor receptor 2 (HER2)-negative), whereas the majority of BRCA2-associated breast cancers are ER-positive [7,8,9].