The S100A9/MRP14 protein and S100A8/MRP8 belong to the low molecular mass calcium-binding S100 proteins, they are composed of two distinct helix-loop-helix motifs (EF-hands) flanked by hydrophobic regions at either terminus and separated by a central hinge region. In human S100A9/MRP14 is usually co-expressed with S100A8/MRP8. Both proteins are expressed during myeloid differentiation, are abundant in granulocytes and monocytes, and form heterodimeric complexes. Although a number of possible functions for S100A8-A9 heterocomplex, including antimicrobial activity, have been proposed, the exact role of these proteins in cell metabolism is still unclear. In human, they have been associated with several inflammatory diseases: phagocytes expressing S100A9 belong to the early infiltrating cells and dominate acute inflammatory lesions; in addition, elevated serum levels of S100A8 and S100A9 have been found in patients suffering from a number of inflammatory disorders including cell arteritis, cystic fibrosis, rheumatoid arthritis, dermatoses, chronic inflammatory bowel disease, chronic bronchitis, some malignancies and autoimmune diseases. Both proteins are localized predominantly in the cytoplasm. An increase in the intracellular calcium concentration leads to a translocation to cytoskeletal components and to the plasma membrane. In addition, it could be demonstrated with human monocytes that both proteins are secreted by an energy-consuming pathway which is dependent on an intact microtubule network and involves protein kinase C.