Tropomyosin is a dimeric coiled coil protein that binds along the length of actin filaments. It is associated with the thin filaments of muscle cells and the microfilaments of nonmuscle cells. Chicken embryo fibroblasts (CEF) contain five isoforms of tropomyosin (a, b, 1, 2, and 3), identified as such by their different apparent molecular masses after separation by SDS-PAGE, but similar biochemical properties, such as resistance to heat and organic solvents, the ability to bind to F actin filaments, and the lack of proline and tryptophan.