Actin is a 43kD highly conserved structural protein. There are three main actin isotypes (alpha,beta and gamma) which show >90% amino-acid (aa) homology between isotypes and >98% homology within members of a particular isotypic group. The majority of the isotype heterogeneity is located in the amino-terminal 30aa. The different actin isotypes have been shown to behave very differently in vitro and in vivo. Recent studies describe differential subcellular localization of gamma-actin and isotype specific binding of actin associated proteins. There are at least six actin isoforms in vertebrates: cardiac alpha-actin and skeletal-alpha actin (expressed in striated muscle), vascular alpha-actin and enteric gamma-actin (expressed in smooth muscle), and cytoplasmic beta- and gamma-actins.