BRCA1 and breast cancer: In addition, most medullary carcinomas (77%) do not display a mutation in BRCA1. Human breast cancers in which the BRCA1 gene is mutated generally contain a high proportion of proliferating cells that lack ER, PR, and HER2, and express basal CKs (CK5/CK14 and CK17), but not CD10 or SMA.