S100A9 and rheumatoid arthritis: These results suggest that S100A8 and S100A9 proteins may be secreted mainly as the heterodimer from inflammatory cell infiltrates, such as neutrophils and monocytes/macrophages, in the joints of patients with active RA; however, these proteins are thought to be secreted also as the homodimer, because the expression pattern of S100A8 and S100A9 has been shown to differ in vivo, depending on cell types, cell differentiation, and inflammatory conditions [6,22,26].