Myosins () comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other motility processes in eukaryotes. They are responsible for actin-based motility. The term was originally used to describe a group of similar ATPases found in the cells of both striated muscle tissue and smooth muscle tissue. Following the discovery by Pollard and Korn (1973) of enzymes with myosin-like function in Acanthamoeba castellanii, a large number of divergent myosin genes have been discovered throughout eukaryotes. Thus, although myosin was originally thought to be restricted to muscle cells (hence (s) + ), there is no single "myosin" but rather a huge superfamily of genes whose protein products share the basic properties of actin binding, ATP hydrolysis (ATPase enzyme activity), and force transduction. Virtually all eukaryotic cells contain myosin isoforms. Some isoforms have specialized functions in certain cell types (such as muscle), while other isoforms are . The structure and function of myosin is strongly conserved across species, to the extent that rabbit muscle myosin II will bind to actin from an amoeba.