S100A9 is a member of the S100 family of calcium-binding proteins, involved in the regulation of angiogenesis, vascular inflammation, cell survival, motility and invasion. S100A9 complexes with S100A8 to regulate myeloid cell function by activating the toll-like receptor TLR4. Dysregulation of S100A9 is associated with cystic fibrosis and it is also a marker for inflammatory diseases such as rheumatoid arthritis. In cancer, it is upregulated in oral, prostate, lung, cervix, stomach and breast carcinomas and it is a marker of poor prognosis for oral cancer. In immunohistochemistry of normal tissue, S100A9 is primarily positive in monocytes, granulocytes, myeloid derived suppressor cells and other immature cells of myeloid lineage, and it also expressed in squamous epithelia. References: J Clin Diagn Res 2016 10(6):ZE10; Nat. Med. 2007. 13 (9): 1042–9, PMID 17767165; Oncotarget. 2015 Sep 29; 6(29): 28401–28424