Caldesmon is a smooth muscle and nonmuscle regulatory protein that interacts with actin, myosin, tropomyosin, and calmodulin. Smooth muscle caldesmon is an elongated molecule with a calmodulin, tropomyosin, and action-binding region at the C-terminus and a myosin binding domain at the N-terminus. Caldesmon stimulates actin binding of tropomyosin which increases the stabilization of actin filament structure. In muscle tissues, caldesmon inhibits the actomyosin ATPase by binding to F-Actin. Caldesmon is a useful marker for smooth muscle tumors.