MKI67 and neoplasm: Accordingly, the WHO 2017 classification system divides neuroendocrine tumors (NETs) into two basic groups: well-differentiated neoplasms (NET G1 and NET G2, with a proliferation index below 20%) and highly heterogeneous, poorly differentiated neoplasms (NET G3 with a Ki-67 proliferation index above 20%), referred to as neuroendocrine carcinomas (NECs) [11].