Human S100A9 (also MRP-14, Calgranulin-B, and p14) is a 14 kDa member of the S100 family of EF-hand calcium-binding proteins. It is 114 amino acids (aa) in length and contains short sequential modules. There is an N-terminal helical region, followed by a calcium-binding EF-hand domain, two more helical regions, a second EF-hand domain, and three additional helical regions. S100A9 will noncovalently heterodimerize with S100A8. In the presence of calcium, this heterodimer will form a heterotetramer. S100A9 is expressed in granulocytes, monocytes, and macrophages during acute and chronic inflammation. Human S100A9 shares 62% and 57% aa identity with rat and mouse S100A9, respectively.