MRP-14, also known as S100A9, belongs to the S100 family of proteins containing 2-EF-hand calcium-binding motifs. It forms non-covalently associated complexes with another S100 family member, MRP-8. MRP14 is expressed in neutrophils and monocytes; it also has been found in ileal tissue eosinophils. The S100A8/A9 complex has been shown to play a role during calcium-dependent activation as well as have antibacterial, antifungal, immunomodulating, and anti-proliferative effects. It also functions as a chemoattractant factor for neutrophils and recently has been considered an endogenous activator of Toll-like receptor 4. A recent report has identified S100A9 as a marker forCD14(+) HLA-DR(-/low) MDSCs.