Schwannoma is predominantly composed of Schwann cells which are consistently strongly positive for S-100 protein. SOX10 (nuclear stain) is another useful marker for Schwannomas. These two markers can be extremely helpful in making the correct diagnosis in schwannomas with extensive degenerative changes (Ancient Schwannomas) where the neoplastic cells are sparse and may be obscured by myxoid change or hyalinization. They are also helpful in distinguishing deep-seated schwannomas (e.g. retroperitoneum) from well-differentiated leiomyosarcomas. Image courtesy of: Dr. Luciano de Souza Queiroz, Dept. of Pathology, Faculty of Medical Sciences, State University of Campinas (UNICAMP), Campinas, São Paulo State, BRAZIL. Additional images are here.