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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 are involved in the regulation of a number of cellular processes such as cell cycle progression and differentiation. S100A9 may function in the inhibition of casein kinase, and altered expression is associated with the disease cystic fibrosis. Overexpression of S100A9 has also recently been linked to a poor prognosis with invasive ductal carcinoma of the breast.