Protein S100 beta belongs to 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. Among brain cells, S100 beta is often used as a marker for astrocytes. The 2 subunits of S100 protein are the products of separate genes. The beta subunit is present in all S100 positive cells and tumors whereas the alpha subunit is detectable only in specific tissues such as heart, skeletal muscle and brain. The S100 protein is a homodimer composed of two alpha subunits (S100a) or two beta subunits (S100b) or as a heterodimer with one alpha and beta subunit (S100ab or S100ao).