Receptor expression of CD34⁺ progenitors and mature human mast cells. Mature mast cells were grown for 8 weeks in 100 ng/mL SCF. Cells were stained with anti–IL-4R-PE, anti–IL-5R-PE, and anti–Kit-PE and were analyzed by flow cytometry. For IFN-γR, cells were stained with anti–IFN-γR (α chain) mAb and then with antirabbit-PE. Results shown were verified in the examination of 3 separate cultures from separate donors. No cell growth occurred in the absence of SCF.