MSI1 and irritable bowel syndrome: The enteroendocrine cells found in the terminal ileum are peptide YY (PYY), pancreatic polypeptide (PP), enteroglucagon cells, and serotonin.7,21 A study hypothesized that the observed decrease in ileal endocrine cell density among the current patients with IBS could be attributed to reductions in Msi-1 and NEUROG3 cells.