MKI67 and neuroendocrine carcinoma: The WHO 2022 classification of GEP-NENs categorizes them into three groups based on the differentiation, mitotic rate, and Ki-67 index: well-differentiated neuroendocrine tumors (low-grade or G1 NET), well-differentiated neuroendocrine tumors (intermediate-grade or G2 NET), and poorly differentiated neuroendocrine carcinomas (high-grade or NEC).