Mondo Description A well-differentiated neuroendocrine tumor arising from the digestive system. It is characterized by the presence of cells with features similar to those of the normal endocrine cells of the digestive system. The neoplastic cells express immunohistochemical evidence of neuroendocrine differentiation and hormones. There is mild to moderate nuclear atypia and less than 20 mitoses per 10 HPF. It includes well-differentiated endocrine tumors or carcinoid tumors and well-differentiated endocrine carcinomas.