Hyper-IL-6: a highly active designer cytokine consisting of IL-6 and soluble IL-6R. Molecular model of the fusion protein consisting of IL-6 (gray) and sIL-6R (yellow) fused by a flexible peptide linker (green). A, B, C, and D denote the four helices of IL-6; D-II and D-III are the two cytokine-binding receptor domains of the sIL-6R used for the construction of the fusion protein.