There are over 21 different S-100 proteins, and they have intracellular and extracellular functions, including regulation of protein phosphorylation, enzyme activities, cell growth and differentiation, and the inflammatory response. Dysregulated expression of several S100 proteins is a common feature of human cancers, with each type of cancer showing a unique S-100 protein profile and/or isoform. S-100B is a diagnostic marker for melanoma in human and veterinary medicine, and it may also aid the detection of micrometastatic disease in the lymph nodes. Circulating S-100B is measured by two-site sandwich ELISA assays.