SMTN (Smoothelin) is a structural protein that is found exclusively in contractile smooth muscle cells. It associates with stress fibers and constitutes part of the cytoskeleton. SMTN is considered essential for proper contractility of intestinal smooth muscle, and dysfunction may contribute to the pathogenesis of chronic intestinal pseudo-obstruction and colonic intertia. In immunohistochemistry, SMTN has high cytoplasmic positivity in smooth muscle cells and is present in the gastrointestine, esophagus, endometrium, seminal vesicle, and at lower levels in other tissues throughout the body. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; Gastroenterology. 2005 Nov;129(5):1592-601, PMID: 16285958; Appl Immunohistochem Mol Morphol 2013;21:452–459, PMID: 23060305;