A case of INI1-deficient tumor in the forearm successfully diagnosed as epithelioid malignant peripheral nerve sheath tumor: Intratumoral nerve fibers with Wallerian degeneration as a diagnostic aid (e,f) Immunohistochemically, the tumor cells are diffusely and strongly positive for S-100 protein (e) and negative for INI-1 (f). gl Immunohistochemically, the tumor cells were diffusely and strongly positive for S-100 protein (Fig. Malignant peripheral nerve sheath tumor (MPNST) is a spindle cell sarcoma with features suggesting Schwannian differentiation and is often associated with neurofibromatosis type 1 (NF1) or irradiation. [Extracted from the article]