SMN1 and fibromatosis: However, these tumors can be defined better on immunohistochemistry: c-kit is positive only in GISTs; SMA is positive in leiomyomas, fibromatosis, and GISTs (30–40%); desmin is positive in leiomyomas but very rarely in GISTs and fibromatosis; S-100 is negative in fibromatosis and very rarely positive in GISTs or leiomyomas, but positive in schwannomas.