In general, NENs can be classified as follows: well-differentiated G1 neuroendocrine tumors (NETs), with Ki67 ≤ 2% (NETs G1); well-differentiated G2, with Ki67 ranging from 3%–20% (NETs G2); well-differentiated G3, with Ki67 > 20% (NETs G3); and poorly differentiated neuroendocrine carcinomas (NECs), which are high-grade by definition [2]. The gene discussed is MKI67; the disease is neuroendocrine carcinoma.