Cytoplasmic actins, which belong to the microfilament system of cytoskeleton proteins, are some of the most conserved eukaryotic proteins being expressed in mammals and birds. The actin protein consists of six isoforms, varying in their amino acid sequence, but all having the same molecular mass of 42 kDa. The isoforms show more than 90% overall sequence homology, but only 50-60% homology in their 18 Nterminal residues. The N-terminal region appears to be a major antigenic region. There are different α isoforms specific for muscle tissues, i.e. skeletal muscle α, cardiac muscle α, and smooth muscle α, respectively. The β- and γ-actins may be present in muscle cells as well as most other cell types in the body, including non-muscle cells.