Mammosphere cell viability. Adherent MCF7 cells and mammospheres were plated as triplicates in 96-well plates, at a density of 5,000 cells/well and allowed to grow. Cell viability was determined every 24 hours by the MTT viability assay (black bars). Alternatively, adherent MCF7 cells and mammospheres were cultured at a density of 106 cells\plate and allowed to grow. Cells were collected every 24 hours and suspended with 1 ml of complete medium, after which 100 μl were transferred to 96-well plates, in triplicates. Cells were allowed to adhere overnight and cell viability was evaluated by the Neutral Red viability assay (white bars). Results represent means ± SE of at least 3 experiments. Statistical significance was determined by the student’s t-test analysis, compared to the adherent control.