Cytotoxic effect of CPT and etoposide on intact mammospheres. Mammospheres were collected after 5–7 days in culture and total number of cells was evaluated. Adherent MCF7 cells and intact mammospheres were seeded in 96-well plates at a density of 5,000 cells/well. The parental MCF7 cells were allowed to adhere over night, after which cells were treated with various doses of CPT (A, B) or etoposide (C, D) for 24 (A, C) or 48 hours (B, D). Cell viability was examined by the MTT viability assay. Results for CPT represent means ± SE of 2 experiments. Results for etoposide represent means ± SE of 3 experiments. Statistical significance was determined by the student’s t-test analysis: *p < 0.05.