K cells and L cells, the most widely studied EECs, are the targets of antidiabetic therapies because of their secretion of gut peptides [e.g., gastric inhibitory peptide (GIP), glucagon-like peptide-1 (GLP-1), and peptide YY (PYY)] with anti-obesity and/or antidiabetic action. This evidence concerns the gene PYY and obesity due to melanocortin 4 receptor deficiency.