MKI67 and neuroendocrine carcinoma: The basic classification of NETs is based on the mitotic count and Ki-67 index, and is classified into three grades (G1, G2, and G3) that can be applied to almost any part of the GI tract: well-differentiated NETs with benign or uncertain behavior, well-differentiated neuroendocrine carcinomas with low-grade malignancy, and poorly differentiated neuroendocrine carcinomas.