Perineuriomas, a rare entity of peripheral nerve sheath tumors, in contrast to schwannomas or neurofibromas, are stained positive for epithelial membrane antigen (MUC1/CD227), but not for S100 protein; positive staining for neuron filament protein demonstrates axon fibers surrounded by layers of perineural tumor cells [717]. The gene discussed is S100B; the disease is plexiform neurofibroma.