Functional characterization of the hIL-6 mutant proteins. Amino acid substitution is indicated by a single letter code. (−) uninjected oocyte-conditioned medium. Binding to shIL-6R is presented as percentage of the value obtained with WT hIL-6. Biologic activity was measured using human XG-1 myeloma and murine 7TD1 hybridoma cell growth stimulation assays. D₅₀ represents the effective concentration giving half of the maximal response obtained with the WT hIL-6. Values represent means of at least two independent experiments performed in duplicate. Low activity (LA) represents maximal activities below the D₅₀ of the WT. Values in parenthesis were calculated by extrapolation when some activity was still detectable. NA, no activity.