ERBB2 and breast carcinoma: Many experts assume that given the equal proportion of BRCA1 versus BRCA2 mutated breast cancers, and approximately 20% of BRCA1 and 80% of BRCA2 PV are associated with ER+ breast cancers, 50% of all BRCA1/2 mutated breast cancers are of ER+/HER2- subtype.