TAC1 and neoplasm: Serotonin-secreting pNETs may originate as de-differentiated tumours with a mixed cellularity, showing a co-secretion of other substances/hormones (i.e., 5-HT, calcitonin, gastrin, substance P, neurokinins, etc.), or from pancreatic enterochromaffin cells well differentiated in 5-HT production.