Myosin, Smooth Muscle is a critical structural component of the contractile apparatus in smooth muscle cells, and belongs to a large family of motor proteins known as myosins, which function in actin-based motility. Anti-Myosin, Smooth Muscle stains human visceral and vascular smooth muscle cells, as well as human myoepithelial cells. By staining the intact myoepithelial cell layers in benign and in situ malignant breast and bronchoalveolar lesions, Anti-Myosin, Smooth Muscle is useful for differentiating between benign and malignant tumors. In difficult cases, Anti-Myosin, Smooth Muscle is clinically useful in distinguishing benign sclerosing breast lesions and infiltrating carcinomas, due to its strong staining of the myoepithelial layer in the benign lesions versus its negative staining in the infiltrating carcinomas.