In conclusion, although Ki67 proliferation index is not officially included as a diagnostic marker to differentiated adrenal cortical adenoma from carcinoma in the 5th WHO classification of endocrine and neuroendocrine tumors, it should be evaluated and considered a continuous numeric value to give a prognostic indication and to select patients to undergo mitotane therapy. This evidence concerns the gene MKI67 and neuroendocrine neoplasm.