MKI67 and neuroendocrine carcinoma: According to the WHO classification, NEN can be stratified based on their histological differentiation into low- (grade 1; G1), intermediate- (grade 2; G2), and high-grade (grade 3; G3) neuroendocrine tumors (NET) and poorly differentiated neuroendocrine carcinoma (NEC), featuring a highly elevated Ki67 proliferative index and/or mitotic rate [1].