Among the six actin isoforms described in mammals, two are found in virtually all cells (?- and ?-cytoplasmic), two are detected in smooth muscle cells (?- and ?-smooth muscle) and two are present in striated muscles, one predominantly in skeletal (?-skeletal) and one in cardiac (?-cardiac) muscle cells. These actin isoforms differ slightly in their N-terminus, but the sequence of each of these actins is highly conserved in higher vertebRates. Skeletal ?-actin may also be expressed in the mammalian heart. Although during the late stage of fetal development ?-cardiac actin becomes the main isoform in the myocardium, ?-skeletal actin might still make up for almost half of its total striated muscle actin content.