BRCA1 and breast cancer: As the majority of BRCA1-deficient tumors contain a fraction of cells that are positive for VIM and/or other mesenchymal markers, and only a small number of human BRCA1-deficient tumors, i.e. metaplastic breast cancers, are predominantly composed of mesenchymal/VIM-positive cells [16–19, 24, 39, 44, 46], we therefore focused on type 1 p18−/−;Brca1MGKO tumor cells.