Competitive binding assay showing indistinguishable high-affinity binding kinetics for COS cells transiently expressing IL-3Rα + hβc (○) or IL-3RΔCT + hβc (□). Cells were treated with 150 pmol/L ¹²⁵I-IL–3 and a titration of cold competitor was performed. Data are the mean of triplicate points.