S100 protein, so called because of its solubility in 100% saturated ammonium sulphate, is an acidic cytoplasmic protein specific for the nervous system. It is localized primarily to glial elements of the brain, Schwann cells in the peripheral nervous system and satellite cells in sympathetic ganglia. The S100 family of calcium binding proteins contains approximately 16 members each of which exhibits a unique pattern of tissue/cell type specific expression. This protein is found in most melanoma cells; therefore, this test is useful in the diagnosis of metastatic melanoma, and as a marker for monitoring treatment. ELEVATED blood levels are mostly seen in patients with metastatic melanoma. It has also been observed in Alzheimer’s disease, AIDS, minor head injuries, acute stroke, cardiac arrest, and cardiac surgery.