S100B is a Ca2+-binding protein, which is secreted from astrocytes. S100B is widely expressed in astrocytes, certain neuronal populations, Schwann cells, myeloid-derived cells, and a few other cell types. In diagnostic pathology, S100B – also termed “S100" – is used as a marker for Schwann cells and melanocytes. In these cell types, S100B is highly expressed, however, S100B is also regularly seen in various other cell types. It is mainly used for differentiated diagnosis of malignant melanoma, cartilagea, and neurological tumors.