Actin is a major component of the cytoskeleton and is present in every cell type. Actin can be resolved on the basis of its isoelectric points into three distinctive components: alpha, beta, and gamma in order of increasing isoelectric point. Smooth Muscle Actin antibody does not stain cardiac or skeletal muscle; however, it will stain myofibroblasts and myoepithelial cells. This antibody could be used together with anti-Muscle Specific Actin to distinguish leiomyosarcoma from rhabdomyosarcoma. In most cases of rhabdomyosarcoma, this antibody gives negative results whereas anti-Muscle Specific Actin is positive in the rhabdomyoblasts. Leiomyosarcomas are positive with both anti-Muscle Specific Actin and anti-Smooth Muscle Actin antibodies.