S100A9 is a member of the S100 family of proteins containing 2 EF hand calcium-binding motifs. S100 proteins are localized in the cytoplasm and/or nucleus of a wide range of cells, and involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. S100 genes include at least 13 members which are located as a cluster on chromosome 1q21. This protein may function in the inhibition of casein kinase. Intracellular S100A9 alters mitochondrial homeostasis within neutrophils. As a result, neutrophils lacking S100A9 produce higher levels of mitochondrial superoxide and undergo elevated levels of suicidal NETosis in response to bacterial pathogens. Furthermore, S100A9-deficient mice are protected from systemic Staphylococcus aureus infections with lower bacterial burdens in the heart, which suggests an organ-specific function for S100A9.