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. The gene discussed is MMP2; the disease is rheumatoid arthritis.