Loss of Pak1 does not affect expression of important mast cell maturation markers. Mast cells were cultured for 4 weeks, and expression of c-kit and FcϵRI was measured by incubation with anti-IgE followed by FITC-conjugated anti–mouse IgG, as well as PE-conjugated anti–c-kit antibodies. Double-positive cells (upper right quadrant) are mature mast cells, expressing both c-kit and FcϵRI. Data shown are representative of 6 independent lines from each genotype.