Actin is part of the cytoskeletal system of all cell types. Smooth Muscle Actin belongs to the actin family of proteins, which are highly conserved proteins that play a role in cell motility, structure and integrity. Alpha, beta and gamma actin isoforms have been identified, with alpha actin being a major constituent of the contractile apparatus, while beta and gamma actins are involved in the regulation of cell motility. Actin filaments interact with myosin to assist in muscle contraction as well as aiding in cell motility and cytokinesis.

Smooth Muscle Actin or alpha actin is found in myofibroblasts and myoepithelium, but not in cardiac and skeletal muscles. Labelling of smooth muscle actin in concert with muscle specific actin staining can allow for differentiation between rhabdomyosarcoma and leiomyosarcoma, as Muscle specific actin is found in rhabdomyoblasts, while smooth muscle actin is found in leiomyosarcomas.