As such, NENs are now classified as well-differentiated neuroendocrine tumors (NETs)—Grade (G)1, G2, and G3 NET or G3 poorly differentiated neuroendocrine carcinoma (NEC), based on their histopathologic features, differentiation, mitotic count, and Ki-67 proliferation index (Table 1). Here, MKI67 is linked to neuroendocrine carcinoma.