Tyrosine phosphorylation of constitutive hβc mutants in factor-independent FDC-P1 cells. FDC-P1 cells coexpressing hGMR and the indicated β subunits were incubated without (−) or with (+) 50 ng/mL hGM-CSF for 10 minutes. Whole cell lysates were immunoprecipitated with an anti-hβc antibody and immunoblotted using (A) an antiphosphotyrosine antibody and (B) an anti-hβc antibody. The position of hβc in each panel is indicated by an arrow.