The tropomyosins are a family of actin filament binding proteins. These proteins were first isolated from skeletal muscle, but later identified in many nonmuscle tissues. Tropomyosins are ubiquitous proteins of 35 to 45 kD associated with the actin filaments of myofibrils and stress fibers. Vertebrates have at least 4 different tropomyosin genes; in humans, they are named TPM1, TPM2, TPM3, and TPM4. Tropomyosins expressed as different isoforms in muscle and non-muscle cells.