S100 calcium binding protein A5, also known as S100A5, is a member of the S100 family of proteins containing 2 EF-hand calcium-binding motifs. S100 proteins are localized in the cytoplasm and 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. S100A5 has a Ca2+ affinity 20- to 100-fold higher than the other S100 proteins studied under identical conditions.