What Is TB-500? Thymosin Beta-4 vs. the TB-500 Fragment, Explained
By the Zen Well Labs Research Team · Last reviewed September 2026 · For laboratory research reference only
Short answer: “TB-500” is a trade name, not a chemical name, and vendors use it for two different molecules: full-length thymosin beta-4 (Tβ4), a 43-amino-acid peptide of about 4963 Da with CAS number 77591-33-4, and a short N-acetylated 7-residue fragment (Ac-LKKTETQ, about 889 Da) taken from Tβ4’s actin-binding region. The only reliable way to know which one a vial contains is the mass reported on its Certificate of Analysis. The TB-500 listed by Zen Well Labs is specified as full-length thymosin beta-4 and is sold for in-vitro research only.
TB-500 at a glance
| What the name refers to | A trade name applied to synthetic thymosin beta-4 or to its Ac-LKKTETQ fragment |
| Full-length thymosin beta-4 | 43 amino acids, N-terminally acetylated |
| Tβ4 molecular formula | C212H350N56O78S |
| Tβ4 molecular weight | ≈4963.5 Da |
| Tβ4 CAS number | 77591-33-4 |
| Fragment (17–23) | Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), ≈889 Da |
| Typical supplied form | Lyophilized powder |
| Regulatory status | Not FDA-approved for any use; listed on the WADA Prohibited List |
What is thymosin beta-4?
Thymosin beta-4 is a small, highly conserved protein found in most mammalian cell types. It was first isolated from calf thymus — hence the name — but its best-established cellular role has nothing to do with the thymus: Safer, Elzinga and Nachmias showed in 1991 that Tβ4 is the cell’s principal G-actin-sequestering peptide. By binding actin monomers one-to-one, it buffers the pool of unpolymerised actin and so influences cytoskeletal dynamics and cell motility.
Why are there two different “TB-500” molecules?
Actin binding in Tβ4 maps to a short central motif, LKKTETQ (residues 17–23). Work summarised by Sosne and colleagues (FASEB Journal, 2010) attributed different activities in cell and animal models to different short segments of the protein, including this motif. A synthetic, N-acetylated version of that seven-residue segment was later commercialised — originally in the veterinary and racing world — under the name TB-500, and anti-doping laboratories have published methods that detect that specific fragment.
Over time the name drifted: many research-chemical suppliers now label full-length synthetic Tβ4 as “TB-500.” The two are not interchangeable reagents. They differ more than five-fold in mass, so equal milligram amounts contain very different molar amounts, and literature findings on the full-length protein cannot be assumed to apply to the fragment or vice versa.
How to tell which one you have
- Check the mass spec line on the COA. An observed mass near 4963 Da is full-length Tβ4; a mass near 889 Da is the Ac-LKKTETQ fragment.
- Check the CAS number. 77591-33-4 is thymosin beta-4.
- Check the formula. C212H350N56O78S corresponds to the full-length peptide.
- If a supplier cannot tell you which molecule is in the vial, treat that as a quality red flag.
What has the published research looked at?
Tβ4 has a substantial peer-reviewed literature, reviewed by Goldstein, Hannappel and Kleinman (Trends in Molecular Medicine, 2005). Preclinical themes include actin dynamics and cell migration, angiogenesis in endothelial models, corneal and dermal wound models, and cardiac injury models in rodents. Recombinant and synthetic Tβ4 have entered early-phase clinical investigation for specific indications, but no Tβ4 product — and no TB-500 product — has been approved by the FDA.
Storage and handling
Store lyophilized material sealed, dry and dark at −20 °C. Tβ4 contains a methionine residue, which is susceptible to oxidation, so limit air exposure and repeated freeze–thaw of solutions; aliquot after reconstitution. General principles are covered in our peptide storage and stability guide.
Frequently asked questions
Is TB-500 the same as thymosin beta-4?
Not necessarily. TB-500 is a trade name. Some suppliers use it for full-length thymosin beta-4 (43 amino acids, about 4963 Da) and others for a seven-residue fragment, Ac-LKKTETQ (about 889 Da). The mass on the Certificate of Analysis tells you which one a given vial contains.
What is the CAS number and molecular weight of thymosin beta-4?
CAS 77591-33-4, molecular weight approximately 4963.5 Da, molecular formula C212H350N56O78S.
What does thymosin beta-4 do in cells?
Its best-characterised role is sequestering G-actin monomers, which regulates actin polymerisation and therefore cell shape and motility.
Is TB-500 FDA-approved?
No. Neither thymosin beta-4 nor the TB-500 fragment is approved by the FDA for any use. Both are named on the World Anti-Doping Agency Prohibited List.
Which form does Zen Well Labs list?
The Zen Well Labs TB-500 product is specified as full-length thymosin beta-4 (CAS 77591-33-4, ≈4963.5 Da) and is sold for in-vitro laboratory research only.
References
- Safer D, Elzinga M, Nachmias VT. Thymosin beta 4 and Fx, an actin-sequestering peptide, are indistinguishable. Journal of Biological Chemistry. 1991;266(7):4029–4032.
- Goldstein AL, Hannappel E, Kleinman HK. Thymosin beta4: actin-sequestering protein moonlights to repair injured tissues. Trends in Molecular Medicine. 2005;11(9):421–429.
- Sosne G, Qiu P, Goldstein AL, Wheater M. Biological activities of thymosin beta4 defined by active sites in short peptide sequences. FASEB Journal. 2010;24(7):2144–2151.
- World Anti-Doping Agency. Prohibited List, section S2.
Related
Research use only. All products sold by Zen Well Labs are intended strictly for in-vitro laboratory research. They are not drugs, foods, cosmetics or dietary supplements, are not approved by the FDA for any use, and are not for human or veterinary use. Nothing on this page is medical advice, and nothing here describes or recommends administration to humans or animals.
