Heterogeneous group of neoplams originating from Kulchitsky and enterochromaffin-like cells.

Most tumors arise in the GI system (eg, carcinoid, gastrinoma), pancreas (eg, insulinoma, glucagonoma), and lungs (small cell carcinoma). Other organs include thyroid (eg, medullary carcinoma), and adrenals (eg, pheochromocytoma).

Cells contain amine precursor uptake decarboxylase (APUD) and secrete different hormones (eg, 5-hydroxyindoleacetic acid [5-HIAA], neuron-specific enolase [NSE], chromogranin A).