Description: Actin is a major component of the cytoskeleton and is present in every cell type. Anti-Smooth Muscle Actin (SMA) does not stain cardiac or skeletal muscle, but it stains myofibroblasts and myoepithel. Used together with Muscle Specific Actin, leiomyosarcoma can be distinguished from rhabdomyosarcoma. In most cases of rhabdomyosarcoma, this antibody stains negative whereas Muscle Specific Actin is positive. Leiomyosarcomas are positive with both Muscle Specific Actin and SMA antibodies.