S100A9 in inflammation
S100A9 is a small (13 kD) cytosolic calcium-binding protein. The protein is mainly expressed in neutrophils and monocytes in human but can also be expressed in other cell types under inflammatory conditions. S100A9 is normally co-expressed with S100A8 and forms S100A8/S100A9 heterodimers. Previous publications had suggested that S100A9 could be expressed without S100A8 in certain conditions. Vario