Factors secreted by benign mammary epithelial cells stimulate clonogenic growth of R2-T1AS breast cancer cells. Graph shows 24-well clonogenic growth assay of R2-T1AS cells in mock-conditioned medium (red bar), or in conditioned media derived from G2B-10A cells (G2B-10A CM, green bar), or G-12A cells (G-12A CM, purple bar), or HuMEC cells (HuMEC CM, brown bar). To produce conditioned media, G2B-10A, G-12A, and HuMEC cells, or no cells (mock) were cultured in serum-free, low-glucose medium for 3 days. Conditioned media were supplemented with 5% FBS at the time of the plating of R2-T1AS cells for the clonogenic growth assay. R2-T1AS cells were plated at a density of 40 cells/well in a 24-well plate, in conditioned media, as indicated. Colonies of 10 cells or more were counted after 5 days. Data represent mean number derived from 8 to 15 independent assays (denoted as "n" in graph labels), with each assay performed in 4 replicate wells. **p < 0.001, *p < 0.05, ns-not significant.