MP1 expression is not required for attachment or survival of ER-negative mammary epithelial cells. Cells were transfected with 40 nM control or MP1 siRNAs for all cell lines except 184B5, where 150 nM siRNAs were used, as described in Materials and Methods. At 48 h cells were photographed, then harvested for counting and extract preparation. (A) Photographs of transfected MDA-MB-231, BT-549, Sk-Br-3, and 184B5 cells. Scale bar = 100 μm. (B) Immunoblots of transfected samples. Numbers represent the relative MP1/Actin ratios. (C) Cell counting and trypan blue exclusion assays were carried out as described in the legend to Figure 2. Error bars represent the mean ± SD of three independent experiments for all samples except 184B5 cells. For this cell line the numbers shown represent the average of two independent experiments.