Classification of neuroendocrine neoplasms according to pathological differentiation and grade according to Ki-67 proliferation index
| Differentiation | Ki-67 (%) | Grade | Classification of neuroendocrine neoplasm (NEN) |
| Well differentiated | <3 | 1 (G1) | NET |
| Well differentiated | 3–20 | 2 (G2) | NET |
| Well differentiated | >20, usually <55% | 3 (G3) | NET |
| Poorly differentiated | Usually >55% | 3 (G3) | NEC |
NEC, neuroendocrine carcinoma; NET, neuroendocrine tumour.