A,B) Cell clustering was carried out with mock and myosin II isoform-silenced cells as indicated. At the end of the incubations, samples were fixed and stained for cell nuclei (Hoechst). Data shown are the appearance of cell clusters from a representative experiment (Bar = 200 mμ) and the averages ± SD of quantitative clustering measurements (cell cluster = ≥10 nucleus in 100 μm2 ) for triplicate samples. C) Using the same samples as in A,B, contraction of attached collagen matrices was determined as the difference in height between control matrices (no cells) and experimental matrices. Data shown are the averages ± SD for triplicate samples. Silencing MyoIIA inhibited cell clustering, whereas silencing MyoIIA or MyoIIB or both together had no effect on matrix flattening.