Mcl-1 knockdown negatively impacts on mammosphere formation by BT474 cells. BT474 cells were transfected with control or the indicated siRNA, as described in Figure 1. One day later, the resulting cells were plated in non-adherent conditions under mammosphere medium, as described in Methods. Mammosphere formation was analyzed by microscopy for (A) and the number of mammospheres formed per dish was enumerated (B) two days later. Data are mean ± se of 3 independent experiments normalized to data obtained with transfection with a control siRNA.