Proliferation and differentiation properties of MECs from MMTV-Cre BRCA1-mutant mice . (A and B) Growth and proliferation properties of mammary epithelial cells (MECs) isolated from mouse mammary tumor virus-Cre recombinase (MMTV-Cre) BRCA1-mutant mice are similar to MECs isolated from human BRCA1 mutation carriers. MECs were harvested and plated as described previously . (A, top) Cultures from wild-type (WT) control mice resulted in round acinar structures, whereas (A, bottom) cultures from MMTV-Cre BRCA1flox/floxp53+/- mice showed complex and irregular features. (B) The overall colony-forming efficiency of murine BRCA1-mutant MECs is increased. Non-tumor-bearing WT and MMTV-BRCA1flox/floxp53+/- MECs were compared at age 7 months, and non-tumor-bearing mice from MMTV-Cre BRCA1flox/WT(heterozygous loss of BRCA1) MMTV-Cre BRCA1 flox/floxwere compared at age 12 to 13 months. (C) Mammary glands from MMTV-Cre BRCA1 flox/floxp53+/- mice contain epidermal growth factor receptor (EGFR) and aldehyde dehydrogenase 1 (ALDH1)-positive acini. Immunohistochemistry for EGFR and ALDH1 was performed on five BRCA1-mutant mammary glands and seven Cre-negative controls. Representative images are shown at × 20 original magnification. (D) Erlotinib is active in suppressing the growth of murine MECs from mice of WT, MMTV-Cre BRCA1 flox/floxor MMTV BRCA1 flox/floxp53+/- background. All MECs were seeded in Matrigel-based cultures and photographed and analyzed for colony formation using SIGNATURE software  after 14 days of culture.