After the terminology of subdividing and classifying NEN has been adapted several times in the last decades, according to the current WHO classification, they are stratified based on their histological differentiation and on the Ki-67 proliferative index into low/ moderate- (grade 1/2, G1/2), high-grade (grade 3, G3) neuroendocrine tumors (NET) and neuroendocrine carcinomas (NEC) (Detjen et al. 2021; Rindi et al. 2018). This evidence concerns the gene MKI67 and neuroendocrine carcinoma.