The S-100 is expressed in benign nevi and malignant melanoma cells. Nearly all benign melanocytic lesions contain S-100 protein which is also observed in over 95% of malignant melanomas.

The antibody reacts with α and β subunits of S-100. Positive results aid in the differential diagnosis of melanomas and nerve sheath tumors from carcinomas. For this purpose, the S-100 antibody is a sensitive marker, but it is not as specific as other antibodies. Therefore, it is recommended that S-100 antibody be used in a panel containing antibodies to keratin, HMB45, and epithelial membrane antigen. The S-100 antibody cannot distinguish between benign and malignant lesions.