S-100 protein derived from brain tissue is an acidic calcium-binding protein. In human brain tissue S-100 protein is mainly presented as 3 isoforms - aa homodimer (S-100aa or S-100a(0)), the bb homodimer (S-100b) and ab heterodimer (S-100a). S-100 is a purified mixture of all forms.

Because of its predominant location in astroglial cells S-100 protein can be used as a sensitive and reliable marker for central nervous system injury. Structural damage of glial cells causes leakage of S-100 protein into the extracellular matrix and into cerebrospinal fluid, further releasing into the bloodstream. Measurements of S-100 protein in patient serum samples are useful in monitoring of traumatic brain injury, ischemic brain damage after circulatory arrests, and in diagnosis and prognosis of clinical outcome in acute stroke.