MKI67 and neuroendocrine carcinoma: In the World Health Organization (WHO) 2010 classification, tumors derived from neuroendocrine cells were collectively referred to as neuroendocrine tumors (NETs) and were classified into NET grade (G)1, NET G2, and neuroendocrine carcinoma (NEC) according to their reproductive activity (i.e., their mitotic count and Ki-67 index), not according to their histology.