Myosin regulatory light chain 5, also known as MYL5, is a hexameric ATPase cellular motor protein. Myosin is composed of two heavy chains, two nonphosphorylatable alkali light chains, and two phosphorylatable regulatory light chains. MYL5 is a regulatory light chain and is expressed in fetal muscle and in adult retina, cerebellum, and basal ganglia. Reconstitution of myosin with regulatory light chain 5 or alkali light chain increases filament velocity to intermediate rates, and readdition of both classes of light chains fully restores the original sliding velocity.