MKI67 and neuroendocrine carcinoma: The WHO (2022) classifies them into well-differentiated neuroendocrine tumors (G1, G2 and G3), poorly differentiated neuroendocrine carcinomas (including large and small cell types), and mixed neuroendocrine-nonneuroendocrine tumors, based on the nuclear fission image and the Ki-67 index (5).