TB-500 is a name used for synthetic thymosin beta-4 supplied for research. Thymosin beta-4 is a 43-residue, N-terminally acetylated peptide and one of the main G-actin sequestering molecules in cells: in 1991 it was shown to be identical to Fx, a platelet peptide that forms a 1:1 complex with actin monomers and inhibits their polymerisation. The name is not used consistently. Doping-control laboratories identified the key ingredient of a veterinary product sold as TB-500 as Ac-LKKTETQ, the N-acetylated 17-23 fragment of thymosin beta-4 that corresponds to its actin-binding region, while other studies use TB-500 to mean synthetic thymosin beta-4. The identity of the supplied material is given on the batch Certificate of Analysis.