WASP is required for normal Kit-induced formation of filopodia. BMMC cultures from wt or WASP-deficient mice were obtained, as described in “Mice, cell lines, cell and bone marrow–derived mast cell culture, and KL stimulation.” Mature BMMCs were analyzed for FcϵRI and Kit surface expression as well as for intracellular expression of WASP by flow cytometry. (A) BMMCs were seeded onto fibronectin-coated slides, and cell morphology in absence or presence of KL (45 minutes, 20 ng/mL) was examined by fluorescence microscopy after phalloidin staining of intracellular actin. The images show representative examples of 4 independent experiments. (B) Filopodia formation in wt BMMCs is indicated by white arrows. A summary of the results is shown in a vertical scatter plot indicating the individual data points of the percentage of cells displaying filopodia formation. Bars indicate the mean values. (C) A total of 1237 wt and 1509 wasp^−/− cells in 4 independent experiments was analyzed. P was calculated using an unpaired Student t test.