MKI67 and neuroendocrine neoplasm: WHO classification system, mainly based on the proliferative index, subdivided these malignancies into well differentiated MENs (Neuroendocrine tumors, NETs; grade (G) 1, Ki-67 index ≤ 2%; or G2, Ki-67 index 3 - 20%), and poorly differentiated MENs (Neuroendocrine carcinomas, NECs; G3, Ki-67 index ≥ 20%) 3-5.