The predominant form of thymosin, thymosin beta4, is a member of a highly conserved family of actin monomer-sequestering proteins. Beta-thymosins are the primary regulators of unpolymerized actin, and are essential for maintaining the small cytoplasmic pool of free G-actin monomers required for rapid filament elongation and allowing for the flux of monomers between the thymosin-bound pool and F-actin. Thymosin beta4 sequesters actin, holding it in a form that is unable to polymerize. Due to its profusion in the cytosol and its ability to bind ATP G-actin but not F-actin, thymosin beta4 is regarded as the principal actin-sequestering protein. Thymosin beta4 binds ATP G- monomeric actin in a 11 complex where G-actin cannot polymerize.