Myosin is primarily class II in smooth muscle. Myosin II contains two heavy chains which constitute the head and tail domains. Each of these heavy chains contains the N-terminal head domain, while the C-terminal tails take on a coiled-coil morphology, holding the two heavy chains together. Thus, myosin II has two heads. In smooth muscle, there is a single gene (MYH11) that codes for the heavy chains myosin II, but there are splice variants of this gene that result in four distinct isoforms.