Actin, Smooth Muscle (1A4)

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. Anti-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 Muscle Specific Actin to distinguish leiomyosarcoma from rhabdomyosarcoma. In most cases of rhabdomyosarcoma, this antibody gives negative results whereas Muscle Specific Actin is positive in the rhabdomyoblasts. Leiomyosarcomas are positive with both Muscle Specific Actin and Smooth Muscle Actin antibodies. 1-5