Myosins are a large family of motor proteins found in eukaryotic tissues. They are responsible for actin-based motility. "The term myosin was originally used to describe a group of similar, but nonidentical, ATPases found in striated and smooth muscle cells." From Pollard and Korn, 1973. Following the discovery by Pollard and Korn 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, "myo"), 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 ubiquitous.