TB-500 and thymosin beta-4 are not the same molecule

Thymosin beta-4 is a 43-residue protein. TB-500 is a seven-residue synthetic fragment of it. Here is the identity of each, the thymosin alpha-1 mix-up, and the status of all three as of September 2026.

By Touchstone PeptidesSeptember 22, 20266 minute read

No. Thymosin beta-4 is a 43-residue, N-acetylated protein encoded by the human TMSB4X gene. TB-500 is a synthetic seven-residue fragment of it, residues 17 to 23, with its own acetyl cap. The two differ by 36 residues and more than four kilodaltons, so a certificate can tell them apart in one line.

What thymosin beta-4 is

Thymosin beta-4 was isolated from thymosin fraction 5, a calf-thymus preparation, by Goldstein's group and characterized in 1982 as a 43-residue peptide with an acetylated serine at the N-terminus. The UniProt entry for the human protein (P62328) shows a 44-residue translation product whose initiator methionine is removed, leaving the mature chain:

SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES

The molecule is acidic, contains one methionine and no cysteine, and carries the N-acetyl group that the original isolation described. PubChem lists the acetylated 43-mer as CID 45382195 with the formula C212H350N56O78S and an average molecular weight of 4963. The CAS registry number, 77591-33-4, sits on PubChem's parallel record for the same molecule under its INN, timbetasin (CID 16132341).

The literature describes it as the major actin-sequestering molecule in eukaryotic cells. It binds monomeric G-actin through a short central motif, LKKTET (residues 17 to 22). That motif is why the fragment exists.

The vial we stock as thymosin beta-4 is listed at molecular weight 4963 and PubChem CID 45382195, which is the full-length parent, not the fragment.

What TB-500 is

TB-500 is a common name, not a pharmacopeial or USAN name. FDA's July 2026 briefing document for its Pharmacy Compounding Advisory Committee gives the identity that the name is understood to carry:

TB-500 (free base) is understood today to be a seven amino acid synthetic fragment of thymosin beta-4 (β4) from amino acids 17 to 23 and an acetyl group at the N-terminal leucine amino acid (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH).FDA, PCAC briefing document on TB-500, July 2026

The fragment is Ac-LKKTETQ-OH: the six-residue actin-binding motif plus one glutamine, with an acetyl cap. FDA gives the free base a molecular formula of C38H68N10O14, a molecular weight of 889.01, and CAS number 885340-08-9; the acetate salt is listed at 949.1. That is about 18 percent of the mass of the parent. PubChem carries the fragment as CID 62707662, filed under its UNII, with the same formula and the sequence Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH.

On the name itself, the same document says:

However, TB-500 is a common name and not a United States Adopted Name (USAN). FDA has encountered multiple salts, and derivatives, including different active moieties, sold commercially under the same common name.FDA, PCAC briefing document on TB-500, July 2026

And in a footnote on marketing: "Several websites state that TB-500 is a synthetic version of thymosin beta-4 and often use the terms interchangeably. However, as noted previously, thymosin beta-4 and TB-500 are not the same substance."

"TB-500" on a label therefore does not tell you which molecule is in the vial. A certificate should state the sequence and a mass-spectrometry identity that matches either 889 (the fragment) or 4963 (the parent). If it matches neither, the name is doing work the paperwork has not done. How to read a certificate of analysis covers what an identity panel should show.

Is TB-500 thymosin beta-4?

One is a fragment of the other. A seven-residue peptide cannot fold the way a 43-residue one does, lacks the parent's N-terminal Ac-SDKP sequence, and has its own mass, CAS number and PubChem record. The literature on the two is also separate: full-length thymosin beta-4 has been the subject of US-sited Phase 3 trials sponsored by ReGenTree under the code RGN-259 (an ophthalmic solution; NCT02974907, completed), while FDA's briefing document found no article in which TB-500 itself had been given to humans, and the only registered TB-500 trial as of September 21, 2026 is a Hudson Biotech Phase 1/2 study in China (NCT07487363).

FDA's document also notes that the fragment "appears to have first been synthesized from thymosin β4 in 2003" and that "a veterinary preparation of TB-500 appeared in 2011." Where LKKTETQ has been studied, the models described are in vitro actin-binding and cell-migration assays and animal models; the non-acetylated and acetylated forms are handled as different substances in that literature.

The other mix-up: thymosin alpha-1

Thymosin alpha-1 is not a fragment of thymosin beta-4 and is not related to TB-500. It comes from a different gene. UniProt P06454 (prothymosin alpha) shows thymosin alpha-1 as residues 2 to 29 of that precursor: 28 residues, N-acetylated, highly acidic. Goldstein's laboratory sequenced it in 1977, five years before thymosin beta-4 was characterized, from the same calf-thymus preparation ("thymosin fraction 5"), which is the only reason the two share the word thymosin.

Its INN is thymalfasin. PubChem CID 16130571 gives C129H215N33O55 and a molecular weight of 3108.3; the CAS number is 62304-98-7. A 2009 review reported approval in over 35 countries; there is no thymalfasin product in Drugs@FDA. The vial we stock as thymosin alpha-1 is that 28-residue peptide.

	Thymosin beta-4	TB-500	Thymosin alpha-1
Source gene	TMSB4X	synthetic fragment of thymosin beta-4	PTMA (prothymosin alpha)
Residues	43	7 (residues 17 to 23 of thymosin beta-4)	28
N-terminus	acetyl-Ser	acetyl-Leu	acetyl-Ser
Formula	C212H350N56O78S	C38H68N10O14 (free base)	C129H215N33O55
Average MW	4963	889.01	3108.3
CAS	77591-33-4	885340-08-9	62304-98-7
PubChem CID	45382195 (timbetasin: 16132341)	62707662	16130571

Regulatory status as of September 21, 2026

TB-500. Not an FDA-approved drug. It was placed in FDA's 503A Category 2 on September 29, 2023 as "Thymosin beta-4, fragment (LKKTETQ)". On April 22, 2026 FDA's page moved it to a "nominated but withdrawn" section because, in FDA's words, the nominations "were withdrawn by the nominators." It is in no category now and is not on the 503A bulks list. FDA nevertheless evaluated it at the July 23 to 24, 2026 PCAC meeting and proposed that neither the free base nor the acetate be added to the list; the committee's vote is non-binding, FDA has published no official tally, and no proposed rule has appeared. WADA's 2026 Prohibited List names it explicitly in section S2.3: "Thymosin-ß4 and its derivatives e.g. TB-500," prohibited at all times; see where research peptides sit on the Prohibited List.

Thymosin beta-4 (full length). No FDA-approved product. Covered by the same WADA S2.3 line. Not on FDA's 503A category lists under its own name.

Thymosin alpha-1. No FDA-approved product. It was already outside Category 2 on FDA's page by September 27, 2024, and the April 22, 2026 page lists it as withdrawn, so it was not part of the 2026 changes. Not named on the WADA 2026 list.

None of this is a clearance for any of the three. A research vial of either is a labelled quantity of a defined molecule sold for laboratory use, as our research use only statement explains.

What to check on a certificate

The stated sequence: 43 residues or seven. If it says only "TB-500", ask.
Mass spectrometry identity: an observed mass near 889 (fragment, free base), 949 (fragment, acetate) or 4963 (parent).
The salt form, because the acetate adds 60 to the fragment's mass.
Net peptide content, since a short, charged fragment carries counter-ions and water that purity alone does not show; see net peptide content.

Sources

For laboratory research use only. Not a drug, not a supplement, and nothing here is a claim about what any of this material does in a person or an animal.

Two things before you come in.