HEK293T cells were transfected with point mutants of p35 and either (A) p40 or (B) Ebi3 and lysates were analyzed for paired cytokines by immunoprecipitation and western blot. Lysates were immunoblotted for GFP as a loading control. Residues at the IL-12 interface (R30, L64, N184, R185, Y189), near IL-12 interface (Y163, R164, H174), opposite face of p35 (Y102, E103, K106, D138, E139, Q142) were selected based on the crystal structure of IL-12. Residues in an unresolved loop of p35 (D44, E46, D47, R50) were also selected. (C) Western blots from at least 3 independent experiments were quantified and are shown as percent of wt IL-12 or IL-35 (mean ± SEM). *p< 0.05, ** p< 0.01, *** p< 0.001