S100A9 is a member of the S100 protein family, typically forming a heterodimer with S100A8. It consists of two EF-hand domains and is primarily located in neutrophils and macrophages. S100A9 is involved in regulating inflammation, cell migration, and the formation of the cellular microenvironment. By binding to receptors such as RAGE and TLR4, S100A9 activates signaling pathways like NF-κB and MAPK, promoting the production of cytokines and chemokines. The expression of S100A9 is tissue-specific, and mutations or dysregulation are associated with various diseases, including inflammatory disorders, infections, and cancer.