According to the 5th edition of the WHO classification, GEP-NEN are now classified based on their morphology and proliferative activity into well differentiated G1 (Ki-67 index ≤ 2%), G2 (Ki-67 index 3–20%) or G3 (Ki-67 index > 20%) neuroendocrine tumors (NETs), poorly differentiated neuroendocrine carcinomas (NECs, Ki-67 index > 20%) and mixed neuroendocrine/non-neuroendocrine neoplasm (MiNEN) (Klimstra et al. 2019). The gene discussed is MKI67; the disease is neuroendocrine neoplasm.