Actins are a major component of the cytoskeleton ubiquitously expresses in all eukaryotic cells. Although actin is one of the most conserved eukaryotic proteins, six isoforms characterized by isoelectric point and amino acid sequence analysis. Four of them represent differentiation markers of muscle tissues and two are found practically in all cells. These six different actin isoforms share >90% sequence homology throughout the entire molecule, but each has a unique sequence in the first 18 residues at the amino terminus. These actins are thought to be involved in the maintenance of contractile activity and other cellular function. Anti-Alpha-Actin (smooth muscle) antibody recognizes smooth muscle actin, no cross-reaction with cardiac or skeletal muscle actin and other non-muscle actins. It specifically labels smooth muscle cells, myofibroblasts and myoepithelial cells. This antibody is a useful tool for smooth muscle cells and smooth muscle-derived tumors.