Cytotoxic effect of a combined treatment with CPT and Gefitinib or etoposide and Erlotinib on intact mammospheres. Mammospheres were collected after 5–7 days in culture and total number of cells was evaluated. Intact mammospheres were seeded in low-attachment 96-well plates at a density of 5,000 cells/well, after which cells were treated with (A) the combination of CPT (0.1 μM or 0.02 μM) and Gefitinib (5 μM or 1 μM), or (B) the combination of etoposide (5 μM or 1 μM) and Erlotinib (1 μM or 0.5 μM) for 72 hours. Cell viability was examined by the MTT viability assay. Results represent means ± SE of 2 experiments. Statistical significance was determined by the student’s t-test analysis: *p < 0.05.