| Classification | WHO Grade | Features |
| Well-differentiated neuroendocrine tumor, grade 1 | G1 | <2 mitoses per 10 HPF Ki67 labeling index <2% |
| Well-differentiated neuroendocrine tumor, grade 2 | G2 | 2 - 20 mitoses per 10 HPF Ki67 labeling index 3% - 20% |
| Poorly-differentiated neuroendocrine carcinoma (small cell carcinoma, large cell endocrine carcinoma), grade 3 | G3 | >20 mitoses per 10 HPF Ki67 labeling index >20% |