Biological assessment of the size-optimized peptides from the selected candidate proteins. (A) The size optimization strategy was designed to obtain peptides resembling three single fingers between 12-25 residues long (see Additional file 3 Fig. S2) from the parent candidate proteins which are 61 residues long (i). The spatial structures of the parent protein (i) and the derived peptide containing Loop I (ii) are given (disulfide bonds are shown in green and the fluorescent green denotes different loop sequences. (B) Assessment of the peptides as agonists of IL-6R. R10-14 (i) and the designed and chemically synthesized peptides 14-L1 (ii) were assessed for their ability to stimulate the proliferation of DS-1 cells in the absence of IL-6 in a dose-dependent manner. (C) Assessment of the peptides as antagonists. R10-13 (i) and the designed and chemically synthesized peptides 13-L1 (ii) were challenged in the IL-6 dependent proliferation of DS-1 cells in a dose-dependent manner.