Serotonin (5-hydroxytryptamine), MW 176, is a catecholamine , a monoamine neurotransmitter, found in abundance in the central nervous system, platelets, brain and spinal cord as well as secreted by enterochromaffin cells (EC or Kulchitsky cells, a type of enteroendocrine cell occurring in the epithelia lining the lumen of the gastrointestinal tract) in the stomach, intestines, appendix and in endocrine cells along the gut and in carcinoid tumors. Platelets and mast cells are rich in serotonin. Serotonin plays important physiological functions at the intestinal level. Human intestinal epithelial Caco-2 cellls functionally express SERT both at their apical and basolateral cell membranes. It is a 630aa residue in human (chromosome 17q11), rat and mouse.