BRCA1 and breast cancer: As mentioned, pre-neoplastic human tissue from BRCA1 mutation carriers harbored an expanded luminal progenitor population whose expression profiles are closely aligned with that of basal tumors arising in BRCA1 heterozygous women.5 Subsequent studies concerning genetic predisposition of progenitor cell transformation and BRCA1/p53-deficient transgenic mouse models further indicate a luminal-to-basal mammary tumor conversion under BRCA1-mutated background.7,58,59