S100 is a multigene family of low molecular weight (Mr between 9 000 and 13 000) Ca2+- binding proteins. The family comprises 19 members that are differentially expressed in many cell types. Thus S100B (previously S100β) is most abundant in glial cells of the central and peripheral nervous system, in melanocytes, chondrocytes, and adipocytes whereas S100A1 (previously S100A/ S100α) is most abundant in cardio-myocytes, slow twitch skeletal muscle cells, salivary epithelial cells, and renal cells. Additionally, S100B is found in tumor cells and subpopulations o neurons, while S100A1 has also been detected in hippocampal neurons. S100A6 is expressed by fibroblasts and smooth and heart muscle cells. Members of the S100 family have been implicated in the Ca2+ dependent regulation of a variety of intracellular activities, e.g. protein phosphorylation, cell proliferation (including neoplastic transformation), and differentiation.