GCG and neoplasm: Well-circumscribed tumor with a fibrous pseudocapsule composed of sheets of small round cells with enlarged nuclei, fine stippled chromatin, and moderately clear to amphophilic cytoplasm staining periodic acid Schiff stain positive. positive for CD 99, while negative for cytokeratin (CK), insulin, glucagon, synaptophysin (SYP), and chromogranin (CHR).