Smooth Muscle Myosin, heavy chain (SMMS-1) is a cytoplasmic structural protein that is a major component of the contractile apparatus of the smooth muscle cells. SMMS-1 is also a myoepithelium-associated protein. Anti-SMMS-1 is a mouse monoclonal antibody to smooth muscle myosin, heavy chain that reacts with human visceral and vascular smooth muscle cells. The antibody also reacts with human myoepithelial cells. 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.