¹²⁵I–IL-4 binding to CHO-K1 cells transfected with IL-13Rα′, IL-4Rβ, and γ_c. cDNA for various receptor chains (2 μg/chain) was transfected in CHO-K1 cells (1 × 10⁶) by using Lipofectamine reagent for 48 hours. For IL-4 binding assay, 1 × 10⁶ cells were incubated with 100 pmol/L of¹²⁵I–IL-4 with or without a 200-fold molar excess of unlabeled IL-4 or IL-13. Binding assays were performed on two different occasions. Cell bound radioactivity was determined as described in Materials and Methods. (A) To determine binding affinity, transfected cells were incubated with 100 pmol/L of ¹²⁵I–IL-4 with or without various concentrations of unlabeled IL-4. Scatchard data were analyzed by the LIGAND program (B). In four experiments, the binding data were fitted with only one site model.