MKI67 and neuroendocrine carcinoma: Given this context, NENs are currently classified as neuroendocrine tumors (NETs) and neuroendocrine carcinomas (NECs).[2] NECs are defined as malignant NENs with poor differentiation and high proliferation rates (Ki-67 > 20% and/or mitotic count >20 per 10 high-power fields).