MKI67 and neuroendocrine carcinoma: The classification of NENs originally included (1) well-differentiated NENs, referred to as neuroendocrine tumors (NETs; grade (G) 1 (Ki-67 < 2%) and G2 (Ki-67 2–20%)), and (2) poorly differentiated NENs, referred to as neuroendocrine carcinomas (NECs, G3) (Ki-67 > 20%) [177,184].