MKI67 and digestive system neoplasm: For the most frequent gastroenteropancreatic (GEP)-NETs, well-differentiated cases can be further categorized according to the 2019 WHO classification of tumors of the digestive system depending on their respective Ki-67 index as G1 (low grade, Ki-67 < 3%), G2 (intermediate grade, Ki-67 3–20%), and G3 (high grade, Ki-67 > 20%), whereas NECs per definition have a Ki-67 index > 20% [6].