MKI67 and pancreatic neuroendocrine tumor: The WHO 2019 classification morphologically distinguishes two groups of pancreatic neuroendocrine neoplasias (PanNENs): well-differentiated tumors or pancreatic neuroendocrine tumors (PanNETs) based on degree of differentiation and Ki67 index, further divided into PanNETs-G1 (Ki67 index ≤ 3%), PanNETs-G2 (Ki67 index between 4% and 20%), PanNETs-G3 (Ki67 index above 20%), and poorly differentiated neoplasms, so called neuroendocrine carcinomas (PanNECs) and mixed adenoneuroendocrine carcinomas (MiNen) [43].