The S100 protein is a low-molecular-weight, acidic and calcium binding protein that in functional form exist in dimers. S100 has two subunits: S100-alpha (94 aa; human chromosome 1) and S100-beta (92 aa; human chromosome 21) that forms as either homodimers (alpha-alpha known as S-100a(0) or beta-beta known as S-100b) or as heterodimers (known as S-100a) of ~21 kDa.

S-100 is a low-molecular weight protein that is found in Schwann cells, melanocytes, glial cells, histiocytes, lipocytes, skeletal and cardiac muscle, chondrocytes, adipocytes, myoepithelial cells, macrophages, Langerhans cells, dendritic cells, and keratinocytes. S-100 is a useful marker for Schwann cell-derived tumors, as well as a number of well-differentiated tumors of the salivary gland, adipose and cartilaginous tissue. Anti-S-100 is used to detect melanomas, histiocytosis X, malignant peripheral nerve sheath tumors, and clear cell sarcomas.