MKI67 and neoplasm: Of note, the 2019 WHO classification has modified the cut-off used for the Ki-67 proliferative index to distinguish grade 1 from grade 2 NETs (Ki-67 above or below 3%), emphasized the distinction between grade 3 NETs (low-grade NETs with a high proliferative rate and Ki-67 above 20%) and NECs which are all high grades poorly differentiated aggressive neoplasms (92).