IGFBP6 and rheumatoid arthritis: In addition, since IGFBP6 is susceptible of proteolysis by matrix metalloproteinase 2 (MMP-2) (6) and PLTs are major producer of MMP-2 (25), the inverse relationship between PLT count and IGFBP6 level may also suggest that proteolysis may be another mechanism leading to low IGFBP6 concentrations in RA SF.