Stat5-deficient mice lack detectable mast cells. (A) An example of alcian blue staining from ear skin of Stat5A/B^+/– and Stat5A/B^–/– littermate animals. Original magnification, × 200. (B) Flow cytometry profile of Kit and FcϵRI expression on peritoneal cells extracted from Stat5A/B^+/– or Stat5A/B^–/– littermate animals at 12 weeks of age. Background staining with control antibodies was approximately 0.05%.