CD44+/CD24- cells associate with a basal phenotype not with tumorigenic potential of human breast cancer cell lines. (a) Luminal, basal or mixed cell morphology can be predicted based on CD44 and CD24 expression. Provided are representative flow cytometry dot plots of CD44 and CD24 expression adjacent to phase-contrast brightfield images of each cell line subtype. Cell lines are composed of luminal-like, CD24+ expressing cells (MCF7 and SUM225); basal-like, CD24- expressing cells (SUM159, SUM1315, and MDA.MB.231); or mixed luminal and basal-like cells and consist of both CD24+ and CD24- populations (human mammary epithelial cells [HMEC] and SUM149). (b) The average percentage of CD44+/CD24- cells in each cell line was determined by flow cytometry. Data are presented as the mean ± standard error of the mean for four separate experiments. (c) The frequency of tumor growth was measured for SUM149, SUM159, and SUM1315 cell lines injected orthotopically in nonobese diabetic/severe combined immunodeficient mice when a tumor was palpable (>3 mm) within 100 to 150 days.