Smooth Muscle Myosin, heavy chain (SMM-HC) is a cytoplasmic structural protein that is a major component of the contractile apparatus of the smooth muscle cells. Expression of smooth muscle myosin is developmentally regulated, appearing early and is specific for smooth muscle development. SM-MHC stains the intact myoepithelial cell (MEC) layers present in benign and in situ malignant breast and bronchioloalveolar lesions and is therefore very helpful in distinguishing between benign and malignant tumors. The antibody reacts with smooth muscle cells and myoepithelial cells, but not with myofibroblasts. It is very helpful in distinguishing between benign sclerosing breast lesions and infiltrating carcinomas in difficult cases since it strongly stains the myoepithelial layer in the benign lesions while it is negative in the infiltrating carcinomas.