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Not a peptide but a 344-residue glycoprotein whose job is to sequester activin and myostatin — the basis of both its physiology and its grey-market reputation.
Also known as: FST · FST344 · follistatin · activin-binding protein · FSH-suppressing protein
Part of the Growth hormone & IGF peptides cluster
Overview
Follistatin is a secreted glycoprotein that binds and neutralises members of the transforming growth factor-β family, chiefly activin and myostatin; the “344” in the market name refers to the full-length 344-residue isoform. It was first identified in 1987 as a follicle-stimulating-hormone-inhibiting substance in ovarian follicular fluid and is now known to be essential for development. No follistatin protein product is approved anywhere; the human studies are early-phase gene-therapy trials.
Source & context
Biological / chemical source: Human FST gene product (UniProt P19883); recombinant protein sold as a research reagent and, in the grey market, as a “peptide”
Primary research or clinical context: Endogenous protein; investigational as gene therapy in muscle-wasting conditions; no approved follistatin product exists
What follistatin is
Follistatin is a secreted glycoprotein, not a peptide in the usual sense: at 344 residues in its full-length isoform it is roughly ten times the size of the peptides that share catalogue space with it. It is recorded in the IUPHAR/BPS Guide to Pharmacology as ligand 4933, in UniProt as P19883, and its structure in complex with activin is deposited in the Protein Data Bank (2B0U). It was first described in 1987 as a follicle-stimulating-hormone-inhibiting substance present in ovarian follicular fluid (Phillips and de Kretser, Frontiers in Neuroendocrinology 1998), and its name records that origin.
Mechanism: a sponge for activin and myostatin
Follistatin has no receptor of its own. It works by binding members of the transforming growth factor-β family and neutralising them, and the great majority of its effects run through its high affinity for activin (Phillips and de Kretser, Frontiers in Neuroendocrinology 1998). Because myostatin — the negative regulator of muscle mass — is also a TGF-β family member, follistatin also binds and blocks myostatin and activin A (Aslam et al., Molecular Medicine 2025). Mice lacking follistatin die soon after birth with defects that include insufficient muscle development and skeletal abnormalities (Patel, International Journal of Biochemistry & Cell Biology 1998).
Physiology beyond muscle
Follistatin turned out to be far more widely distributed than its reproductive origin suggested, with roles in embryogenesis, in the pituitary, and in adult tissues associated with reproduction (Patel 1998). Circulating follistatin is now understood to come substantially from the liver and to be tightly regulated by the glucagon-to-insulin ratio in healthy people, a finding that reframed it as a metabolic signal rather than a spill-over from the gonads (Hansen and Plomgaard, Molecular and Cellular Endocrinology 2016). Studies linking follistatin and the related protein follistatin-like 3 to type 2 diabetes and obesity report contradictory results (Bielka et al., Prostaglandins & Other Lipid Mediators 2023).
Preclinical evidence in muscle
Follistatin's muscle effects have been studied mainly by manipulating its gene. Deleting follistatin specifically in fibro-adipogenic progenitor cells of obese mice reduced follistatin expression in skeletal muscle and white adipose tissue and impaired muscle function, with accelerated muscle atrophy (Aslam et al., Molecular Medicine 2025). The muscle phenotype of follistatin-null mice points the same way. These are genetics results in mice; they do not describe what an injected recombinant protein does in a person.
Human studies: gene therapy, not injections
ClinicalTrials.gov lists 15 registered studies naming follistatin as of September 2026. The interventional ones deliver the gene, not the protein: a completed phase 1 safety study of an injectable follistatin plasmid gene therapy (NCT06411366), a completed early-phase study of combined Klotho and follistatin gene therapy (NCT07285629), and a recruiting phase 1/2 trial of AAV-delivered follistatin against age-related muscle loss (NCT07443826). Several others merely measure follistatin levels. No trial of recombinant follistatin protein as a muscle-building injection has been registered, which is what the grey-market product claims to be.
| Study | Design | Status |
|---|---|---|
| NCT06411366 | Phase 1 — injectable follistatin plasmid gene therapy, safety | Completed |
| NCT07285629 | Early phase 1 — Klotho + follistatin gene therapy | Completed |
| NCT07443826 | Phase 1/2 — AAV-follistatin for age-related muscle loss | Recruiting |
“Follistatin 344” in the grey market
Vendors sell “follistatin 344” as a 1 mg lyophilised vial and describe it as a muscle-growth peptide. Three things are worth knowing. A 344-residue glycoprotein cannot be made by the solid-phase synthesis used for peptides; it must be expressed in cells, so the product's identity and glycosylation depend entirely on the vendor's supplier. No human study of injected follistatin protein exists. And the mechanism that makes it interesting — blocking activin and myostatin — is one with systemic consequences in the gonads, liver and pituitary, none of which has been characterised for an injected product.
Relation to other profiles
Follistatin is the natural antagonist of myostatin, which has its own profile on this site, and it acts on the same pathway that anti-myostatin antibodies target. It is unrelated in mechanism to the growth-hormone secretagogues (sermorelin, ipamorelin, CJC-1295) with which it is usually sold. This profile is educational and is not guidance on obtaining or using any product.
Structure & chemistry
Molecular identity & reference confidence
Reference mass unavailable
Reference values are taken from the cited molecular record or curated profile. A database match is not independent laboratory verification; modifications, counter-ions, and formulations can change the measured mass.
- Molecular form
- Single-chain secreted glycoprotein encoded by the human FST gene (UniProt P19883); crystal structure deposited as PDB 2B0U
- Species
- Not established by this molecular identity audit
- Termini (N / C)
- Not specified / Not specified
Average reference mass
Reference policy updated: 2026-10-05
- IUPHAR/BPS source snapshot · 2026-09-08
- Receptors
- No receptor of its own: acts as a soluble binding protein that sequesters activin, myostatin and other TGF-β superfamily ligands
- Backbone
- Single-chain secreted glycoprotein encoded by the human FST gene (UniProt P19883); crystal structure deposited as PDB 2B0U
- Status
- Endogenous protein; no approved therapeutic product (September 2026)
- IUPHAR ligand: 4933
Chemistry values: curated profile, IUPHAR/BPS Guide to Pharmacology · fetched 2026-09-08.
Change log
Dated observations from the data pipeline (registry flags, trial footprints, identifiers) and editorial reviews of this profile.
- IdentifiersCross-linked to IUPHAR ligand 4933.
- EditorialProfile content reviewed and updated by the editorial team.
FAQ about Follistatin 344
What is Follistatin 344?+
Follistatin is a secreted glycoprotein that binds and neutralises members of the transforming growth factor-β family, chiefly activin and myostatin; the “344” in the market name refers to the full-length 344-residue isoform. It was first identified in 1987 as a follicle-stimulating-hormone-inhibiting substance in ovarian follicular fluid and is now known to be essential for development. No follistatin protein product is approved anywhere; the human studies are early-phase gene-therapy trials.
What Canadian product status is recorded for Follistatin 344?+
Canadian product status for Follistatin 344 has not been verified in this profile. No verified record here does not mean no authorization exists. Status depends on the product, formulation and use; consult Health Canada's product records.
What is the typical length of Follistatin 344?+
Follistatin 344 is commonly described as approximately 344 amino acids (Secreted glycoprotein (activin-binding protein)).
What is follistatin 344?+
The full-length 344-residue isoform of follistatin, a secreted glycoprotein that binds and neutralises activin and myostatin. The market name refers to that isoform.
Is follistatin a peptide?+
No — it is a 344-residue glycoprotein, about ten times the size of a typical research peptide, and it is made in cells rather than by chemical synthesis.
How does follistatin work?+
It has no receptor of its own. It binds TGF-β family proteins, mainly activin and myostatin, and blocks their signalling.
Is follistatin approved?+
No follistatin product is approved anywhere. The only human trials are early-phase gene-therapy studies.
Has injected follistatin been tested in people?+
No. Registered trials deliver the follistatin gene by plasmid or AAV; none has tested the recombinant protein as an injection.
Further reading
- FoundationsPeptides vs SARMs: A Science-Literacy ComparisonAn educational look at how peptides (amino-acid chains acting through receptor and signaling biology) differ from SARMs (non-peptide small molecules targeting the androgen receptor). Framed as science literacy, not usage advice.
- FoundationsWhat Are Research Peptides? A Clear Educational GuideLearn what “research peptides” means, how they differ from approved peptide drugs, quality markers (COA, purity), and how to study them responsibly.
- SafetyPeptide Safety and Risks: A Balanced Educational OverviewAn objective, balanced overview of what research does and does not establish about peptide risks—unknowns, purity and contamination concerns, and why unapproved compounds carry uncertainty. Not reassurance and not medical advice.
- Hormone axisGH Secretagogue Peptides Explained: CJC-1295, Ipamorelin, GHRPsEducational overview of growth hormone secretagogue peptides—GHRH analogs vs GHRPs, DAC vs no DAC, receptors, and research context.
Related peptides
References & further reading
- 1.Patel K (1998). Follistatin. International Journal of Biochemistry & Cell Biology
- 2.Phillips DJ, de Kretser DM (1998). Follistatin: a multifunctional regulatory protein. Frontiers in Neuroendocrinology
- 3.Hansen JS, Plomgaard P (2016). Circulating follistatin in relation to energy metabolism. Molecular and Cellular Endocrinology
- 4.Bielka W et al. (2023). Follistatin and follistatin-like 3 in metabolic disorders. Prostaglandins & Other Lipid Mediators
- 5.Aslam MR et al. (2025). Deletion of fibro-adipogenic progenitors-specific follistatin impairs muscle function and accelerates skeletal muscle atrophy in obese mice. Molecular Medicine
- 6.Robertson DM (1992). Follistatin/activin-binding protein. Trends in Endocrinology and Metabolism
- 7.IUPHAR/BPS Guide to Pharmacology — follistatin (ligand 4933)