MKI67 and neuroendocrine neoplasm: Based on grading (mitotic rate and Ki-67 index) and morphological differentiation, NENs are classified according to the 2022 World Health Organization (WHO) classification system for neuroendocrine neoplasms into well-differentiated neuroendocrine tumors (NETs) of low grade (G1, Ki-67 < 3%), intermediate grade (G2, Ki-67 < 3%), and high grade (G3, Ki-67 > 20%), as well as poorly differentiated NECs (G3, Ki-67 > 20%, often >70%), which are highly aggressive neoplasms with unfavorable outcomes [32].