S100 is a family of cytoplasmic calcium-binding proteins expressed in numerous cell lines which can be targeted by immunohistochemistry. Staining for S100 is helpful in characterizing a number of tumors, including malignant melanoma, glial tumors, neurogenic tumors (e.g. schwannomas and neurofibromas), mesenchymal tumors (e.g. chondromas, chondrosarcomas, liposarcomas, some osteogenic sarcomas) and some histiocytic tumors .

History and etymology

S100 is a contracture of 100% soluble in ammonium sulphate at neutral pH .