We hypothesized the relationship between tumorigenesis and loss of immunoreactive menin in pancreaticoduodenal small NETs in MEN1 as follows: “At the early stage of small NET development, tumor cells exhibit features of neuroendocrine cells, as indicated by the strong positivity for CgA, gastrin, or somatostatin, without loss of menin. Here, GAST is linked to neoplasm.