Biologic activity of hIL-6 mutants F74E, F78E, and R179E on human and murine cells. (A) Growth arrest of human A375 melanoma cells. (B) Growth stimulation of human XG-1 myeloma cells. (C) Growth stimulation of murine 7TD1 hybridoma cells. Values are the mean of at least two independent experiments performed in triplicate, with standard deviation less than 10%. (□) WT; (⋄) F74E; (○) F78E; (▵) R179E.