Calprotectin is a heterocomplex of the S100 proteins S100A8 and S100A9 (also called myeloid-related protein 8 [MRP8] and MRP14), found mainly in circulating neutrophils and monocytes and in macrophages in RA synovial tissue [3, 4]. The gene discussed is S100A8; the disease is rheumatoid arthritis.