MKI67 and neoplasm: According to World Health Organization (WHO) 2017, based on WHO 2010 and European Neuroendocrine Tumor Society (ENETS) classifications that are crucial for the diagnosis of NENs, the histopathological examination should be supplemented with immunohistochemistry, which is based on Ki67 expression and allows for the division of NENs into three main groups of well differentiated tumors: NET G1 (low grade) with Ki67 ≤ 2%, NET G2 with Ki67 3–20% (intermediate grade), and NET G3 with Ki67 > 20% (high grade).