Therefore, in this study, we attempted to explore the followings in GI-NETs: 1) the possible association between SSTR2 immunoreactivity and the embryological origin and the proliferative activity of tumor cells, 2) the correlation between the therapeutic efficacy of SSAs and SSTR2 immunoreactivity in GI-NET patients treated with SSAs, with particular emphasis on immunoreactivity evaluated by DIA, and 3) SSTR2 immunoreactivity in the normal neuroendocrine cell counterparts from which tumor cells originate in order to account for the diversity of SSTR2 immunoreactivity in tumor cells. The gene discussed is SSTR2; the disease is digestive system neuroendocrine tumor, grade 1/2.