FACS profiles of FDCP-mix cells transfected with hIL-3 and hGM-CSF receptor subunits. Cells were labeled with anti–hIL-3 and hGM-CSF receptor subunit antibodies in a 2-step procedure. The black histograms denote nonspecific fluorescence obtained with the secondary fluorescent reagent only and are overlaid with the histograms obtained after labeling with both the primary antireceptor subunit antibody and secondary reagent. Isotype control antibodies gave similar flow profiles to those obtained by using the secondary reagent only (data not shown). Results are shown for expression of the (A and D) hIL-3 R, (B and E) hGM R and (C and F) hβ_c receptor subunits by the (A-C) parental FDCP-mix cells and cells transfected with (D-F) hIL-3 R, hGM R, and hβ_c subunits and are representative of least 6 such experiments using different clones expressing hIL-3 R, hGM R, and hβ_c subunits, respectively. Similar profiles were obtained for cells coexpressing hβ_c subunits and either hIL-3 R or hGM R subunits.