S100B is a calcium binding protein, which is secreted from astrocytes. It is a small dimeric cytosolic protein (21 kDa) consisting of ββ or αβ chains. S100B is involved in a variety of intracellular and extracellular regulatory activities.

Over the last decade, S100B has emerged as a candidate peripheral biomarker of blood–brain barrier (BBB) damage and CNS injury. Elevated S100B levels accurately reflect the presence of neuropathological conditions including traumatic head injury and neurodegenerative diseases. Normal S100B levels reliably exclude major CNS pathology. Serum S100B has also been reported as a useful marker for early detection of metastases of melanoma and cerebral complications from head injury, cardiac surgery, and acute stroke.