MKI67 and neuroendocrine neoplasm: Although most grade 3 tumors have been poorly differentiated NECs (PDNECs), it has been reported that some well-differentiated NENs also demonstrate a proliferation rate in the grade 3 range.5,6,7,8 In light of these reports, the 2017 WHO classification system9 and the 2019 WHO classification system10 proposed 3 grades (1, 2, and 3) for neuroendocrine tumors (NETs)—a new entity of NETs with grade 3 characterized by well-differentiated morphology and a Ki-67 index greater than 20%, separate from PDNECs.