Immunohistochemical characteristics of primary and liver metastases. The initial liver biopsy shows the characteristic cell formation pattern of neuroendocrine tumor cells of solid nests of regular cells with broad eosinophilic cytoplasm and small inconspicuous nuclei. Mitotic cells are absent (a). The proliferation rate is very low, Ki67 marks less than 1% of the tumor cell nuclei (b). Furthermore, the strong immunohistochemical reaction for chromogranin A and synaptophysin is a solid proof for neuroendocrine differentiation (c and d). Likewise the rectum polyp exhibits the same histological (e) and immunohistochemical features including low proliferation (f) and marked reaction for the neuroendocrine markers chromogranin A and synaptophysin (g and h).