Smooth muscle myosin is a primary contractile protein expressed in smooth muscle. It is a common myoepithelial marker in immunohistochemistry, showing cytoplasmic and membranous positivity in myoepithelial and smooth muscle cells. It tends to provide more sensitive results with less myofibroblast cross-reactivity when compared to biomarkers like calponin and smooth muscle actin. It is useful for distinguishing breast cancer subtypes; it is expressed in breast hyperplasia, ductal carcinoma in situ and HER2+/ER-/PR- tumors, and downregulated in triple negative breast cancers. It can be used to illustrate ductal wall stability, as ductal tumor cells usually lack expression of smooth muscle myosin. It is also useful for differentiating benign from malignant lung lesions (bronchioloalveolar carcinoma), and for identifying smooth muscle neoplasms. References: Appl Immunohistochem Mol Morphol. 2010 May;18(3):219-25, PMID: 20065853; World Journal of Clinical Oncology. 2014 Aug; 5.3; 382-392, PMID: 25114853; Scientific Reports 2016 Jan; 6; 35773, doi: 10.1038/srep35773