MKI67 and neuroendocrine carcinoma: According to the updated WHO classification of 2019, GEP-NEN are classified based on morphology and proliferation rate into well-differentiated tumors (NET G1–G2: Ki-67 ≤ 20%; NET G3: Ki-67 > 20%) and poorly differentiated, clinically highly aggressive neuroendocrine carcinomas (NEC, always G3: Ki-67 > 20%) (Lloyd 2017; Nagtegaal et al. 2019).