A slow growing neuroendocrine tumor, composed of uniform, round, or polygonal cells having monotonous, centrally located nuclei and small nucleoli, infrequent mitoses, and no necrosis. The tumor may show a variety of patterns, such as solid, trabecular, and acinar. Electron microscopy shows small secretory granules. Immunohistochemical studies reveal NSE, as well as chromogranin immunoreactivity. Malignant histology (cellular pleomorphism, hyperchromatic nuclei, prominent nucleoli, necrosis, and mitoses) can occasionally be seen. Such cases may have an aggressive clinical course. Gastrointestinal tract and lung are common sites of involvement.