MKI67 and neuroendocrine neoplasm: Although some confusion exists as to how to grade tumors that have a Ki67 index between 2 and 3%, a more recent study validated the Ki67 index criteria for grade 1 and grade 2 neuroendocrine tumors of the WHO classification and found that Ki67 index of 3 percent was the appropriated cutoff separating grade 1 from grade 2 tumors [3].