S100 protein is a 21,000-Da component first isolated from brain by Moore (1965). Immunocytochemical studies demonstrated that S100 protein is produced by a wide variety of normal and neoplastic cells of mesodermal, neuroectodermal, and epithelial origin (Herrera et al., 1988). The S100 molecule is a dimer; thus there are 3 forms of S100: alpha-alpha, known as S-100a(0); alpha-beta, known as S-100a; and beta-beta, known as S-100b. S100 genes are clustered on human chromosome 1q21. S-100 protein expression by primary and metastatic adenocarcinomas.