ESR1 and breast cancer: A comprehensive study published by the Consortium of Investigators of Modifiers of BRCA1/2 (CIMBA) including 4325 BRCA1 and 2568 BRCA2 mutation carriers observed a higher proportion of ERα-negative (78%) and PR-negative (79%) cases and a lower frequency of HER2 overexpression (90%) in BRCA1-associated BC compared to sporadic BC [46].