Muscle actin (ACTA1, ACTA) is a protein that is a major component of the contractile apparatus of the cytoskeleton in muscle tissues. Mutations of the ACTA1 gene result in nemaline myopathy type 3. It is expressed in skeletal muscle, smooth muscle, heart muscle, myoepithelial cells, myofibroblasts, pericytes, and decidual cells. Staining is used to identify skeletal muscle and smooth muscle cells in normal tissues and in disease to distinguish soft-tissue tumors with muscle differentiation (eg, rhabdomyoma, rhabdomyosarcoma, leiomyoma, leiomyosarcoma) from other types of tumors. References: Am J Clin Pathol. 1991;96(1):32-45. PMID: 1712542; Cardiovasc Pathol. 2006;15(2):91-99. doi: 10.1016/j.carpath.2005.11.006; Am J Otolaryngol. 2005;26(3):201-206. PMID: 15858778; Nat Genet. 1999;23(2):208-212. doi: 10.1038/13837