Serum S100β is a low-molecular-weight Ca2+-binding protein composed of two isomeric subunits and is found predominately in astroglial and Schwann cells. But not all astrocytes express S100B. It has been shown that S100B is only expressed by a subtype of mature astrocytes that ensheath blood vessels and by NG2-expressing cells. This protein may function in neurite extension, proliferation of melanoma cells, stimulation of Ca2+ fluxes, inhibition of PKC-mediated phosphorylation, astrocytosis and axonal proliferation, and inhibition of microtubule assembly. S100β normally is low or is not detectable in serum. It is a noninvasive Marker of Blood-Brain Barrier Function and Brain Lesions.