POMC and Zollinger-Ellison syndrome: Functioning tumors primarily include gastrinomas, which cause Zollinger-Ellison syndrome, serotonin-producing tumors associated with carcinoid syndrome, and NETs with ectopic hormone secretion, such as insulinomas, growth hormone-releasing hormone (GHRH)-producing NETs causing acromegaly, and adrenocorticotropic hormone (ACTH)-producing NETs causing Cushing syndrome [7].