BRCA1 inhibition leads to increased colony formation and size that is completely blocked by epidermal growth factor receptor (EGFR) inhibition. Mammary epithelial cells (MECs) expressing either (A and B) control small hairpin RNA (shRNA) or BRCA1-directed shRNA or (C and D) primary MECs isolated from reduction mammoplasties or BRCA1 mutation carriers were seeded and photographed after 10 days in culture. All colony formation was completely suppressed when cells were grown in the presence of erlotinib at 2 μM shown in phase images for each cell type (right). (D) MEC growth is inhibited at concentrations as low as 0.5 μM erlotinib. Cells were seeded at 5,000 cells/well, allowed to grow for 10 days in the presence of the indicated amounts of erlotinib and then counted. (E) Cell viability assay of control and BRCA1 shRNA-expressing human MECs. Cells were seeded in triplicate in 96-well plates at 250 cells/well, and cell viability was determined daily. Dotted line and open symbol represent control cells, and closed symbols represent cells expressing BRCA1-directed shRNA. Black lines, vehicle control; blue lines, culture in the presence of 2.5 μM erlotinib; red line, culture in the presence of 5 μM erlotinib.