S-100 is a marker of schwann cells and melanocytes and therefore is used to evaluate nerve sheath tumors and melanoma. Staining is cytoplasmic and nuclear. It helps differentiate schwannoma (stains all cells) from neurofibroma (mixture of positive and negative cells). It can also be used to identify perineural invasion by malignant tumors. In addition, S-100 stains Langerhans cell histeocytosis as well as myoepithelial cell tumors.