Follistatin 344

Follistatin 344 (FST-344) is a naturally occurring isoform of the human protein follistatin, which binds and neutralizes myostatin and related activin-family signals that limit muscle growth. Nearly all of the meaningful human data on this specific isoform comes from gene-therapy studies, where a gene encoding follistatin-344 is delivered into muscle using a viral vector, not from injecting the follistatin protein itself. Follistatin 344 sold by research-chemical vendors as an injectable peptide has essentially no human pharmacokinetic, dosing, or safety data behind it as an injected molecule.

Research snapshot

Category	Current information
Peptide category	Naturally occurring myostatin/activin-binding glycoprotein isoform
Primary research interest	Muscle mass and function in neuromuscular disease (via gene therapy); vendor/community “anti-myostatin” muscle-growth use
Highest available evidence	Published human clinical trials, but only of AAV gene-therapy delivery, not injected protein
Human research available	Yes, but a category mismatch: human data is from small Phase 1/1-2 gene-therapy trials that cause muscle to produce the protein locally; no human data exists for directly injected follistatin-344 protein
Development status	Not an approved drug in any form; no active injectable-protein clinical development identified
Regulatory status	Not approved anywhere
Last reviewed	September 27, 2026

Technical identity

Technical property	Information
Primary name	Follistatin-344 (FST-344)
Alternative names	Follistatin, FST; related isoforms follistatin-288 and follistatin-315 in some naming conventions
Peptide sequence	Not reliably established as a standardized short-peptide sequence; follistatin-344 is a roughly 344-residue protein isoform, not a synthesized short peptide, and no authoritative, vendor-independent sequence record was verified for this entry. Human follistatin is catalogued at UniProt accession P19883.
Amino-acid length	Approximately 344 residues (this isoform’s namesake length)
Molecular formula	Not reliably established
Molecular weight	Not reliably established
CAS Registry Number	Not reliably established
PubChem CID	Not reliably established
Chemical modifications	Natively glycosylated protein; not a synthetic modified peptide
Peptide class	Activin-binding glycoprotein (follistatin family)
Primary biological target	Myostatin (GDF-8) and related activin-family growth factors, neutralized by binding rather than receptor blockade
Developer or originator	Naturally occurring human protein; studied clinically as a gene-therapy transgene (rAAV1.CMV.huFollistatin344) by Nationwide Children’s Hospital and collaborators
Development status	Only gene-therapy delivery has reached human trials (both completed Phase 1/1-2); no injectable-protein product has been tested in humans

What it is

Follistatin is a naturally occurring, single-chain glycoprotein produced in many human tissues. It works primarily by binding tightly to myostatin (GDF-8) and other activin-family growth factors, blocking their ability to signal through their receptors. Myostatin normally acts as a brake on skeletal muscle growth; by soaking up myostatin, follistatin removes some of that brake, which is why it has attracted interest for muscle-wasting conditions and, separately, for performance and physique-enhancement use outside any medical indication.

Follistatin exists naturally in a small number of isoforms produced from the same gene by alternative processing, most notably a shorter form and a longer form referred to by their approximate amino-acid length — commonly follistatin-288 and follistatin-315 in some naming conventions, and follistatin-344 in others, reflecting differences in exactly how the protein is processed and whether it carries a C-terminal extension. Follistatin-344 is the isoform used in essentially all of the published clinical gene-therapy research referenced in this entry, delivered via a recombinant adeno-associated virus (AAV) vector carrying the human follistatin-344 gene (commonly denoted rAAV1.CMV.huFollistatin344), rather than administered as a manufactured, injectable peptide or protein product.

This distinction matters because the commercial “research peptide” market sells vials labeled “Follistatin 344,” implying an injectable protein product analogous to a peptide hormone, when the actual human research record for this specific molecule is built almost entirely on one-time intramuscular gene-therapy injections that cause the patient’s own muscle cells to manufacture the follistatin-344 protein locally over an extended period — a fundamentally different intervention, dosing concept, and pharmacokinetic profile than injecting the protein directly and repeatedly, which is how vendor products are marketed to be used.

How does it work?

Plain-English explanation

Myostatin acts like a brake pedal on muscle growth: the more active it is, the more it limits how large and strong muscle fibers become. Follistatin works like a hand that grabs the brake pedal and holds it down so it cannot be pressed — by binding directly to myostatin (and some related signals) and preventing it from reaching its receptor on muscle cells. Less braking signal getting through means muscle cells are freer to grow, at least in principle.

Technical explanation

Follistatin binds myostatin (GDF-8) and activin A with high affinity, preventing these TGF-beta superfamily ligands from engaging activin type IIB receptors (ActRIIB) on muscle cells and triggering downstream Smad2/3 signaling that suppresses muscle protein synthesis and satellite cell activity. In the published gene-therapy trials, a recombinant AAV1 vector carrying the human follistatin-344 coding sequence under a CMV promoter is injected directly into a target muscle (commonly the quadriceps), where it drives sustained local overexpression of follistatin-344 protein by the patient’s own muscle fibers, intended to counteract myostatin/activin signaling regionally, at the injection site, rather than systemically.

Potential benefits and research applications

Muscle mass and function in neuromuscular disease (gene therapy)

What is being investigated: whether local, AAV-delivered follistatin-344 gene transfer can increase muscle size and improve function in conditions marked by progressive muscle loss, specifically Becker muscular dystrophy, sporadic inclusion body myositis, and Duchenne muscular dystrophy. How the effect might occur: sustained local myostatin/activin blockade in the injected muscle, promoting muscle fiber growth. Evidence: published human Phase 1 and Phase 1/2 gene-therapy trials (Nationwide Children’s Hospital, PI Jerry Mendell) reported increases in muscle fiber size on biopsy and, in the sporadic inclusion body myositis cohort, improvement in six-minute walk distance. Strength: preliminary and limited — these were small, early-phase, mostly open-label or small-cohort trials (total enrollment in the low teens across studies), not placebo-controlled efficacy trials, and a subsequent published critique specifically disputed whether the reported functional improvements in the inclusion body myositis cohort were adequately supported by the data. Limitation: this evidence applies to gene-delivered, muscle-produced follistatin-344, not to an injected protein product, and does not establish a dose, safety profile, or benefit for injectable follistatin.

Muscle growth and “anti-myostatin” use outside medical indications (vendor/community framing)

What is being investigated: use of injectable “Follistatin 344” peptide products by the research-chemical and physique-enhancement community for muscle growth, fat loss, and recovery, independent of any diagnosed muscle-wasting disease. How the effect might occur: presumed myostatin inhibition, by analogy to the gene-therapy research and to animal myostatin-knockout models, which show dramatic muscle mass increases. Evidence: has been reported anecdotally in community and vendor material; no identifiable human clinical trial of injected follistatin-344 protein for muscle growth in healthy or non-disease populations was found in this research. Strength: unverified — this application depends entirely on extrapolating from gene-therapy research and animal models to a different route of administration and a different population that has not itself been studied. Limitation: injecting a large protein like follistatin-344 does not automatically reproduce the sustained local overexpression achieved by gene therapy, and no published pharmacokinetic data describing what happens after a direct injection of follistatin-344 protein in humans was identified.

What dosage information circulates?

Figures in this section summarize amounts and schedules reported in published research or circulating online. Their inclusion documents what is reported and does not establish that a regimen is verified, safe, effective, or appropriate.

Reported use or research objective	Route reported	Amount reported	Frequency reported	Reported duration	Evidence or source category
Becker muscular dystrophy / sporadic inclusion body myositis (gene therapy)	Intramuscular, one-time AAV vector injection	Escalating vector-particle dose cohorts; not convertible into a protein-injection dose	One-time	Single administration, followed by extended monitoring	Published human clinical trial (NCT01519349)
Duchenne muscular dystrophy (gene therapy)	Intramuscular, one-time AAV vector injection	Escalating vector-particle dose cohorts; not convertible into a protein-injection dose	One-time	Single administration, followed by extended monitoring	Published human clinical trial (NCT02354781)
Directly injected follistatin-344 protein (any use)	Not applicable	No established or reliably sourced dosing information was identified	Not applicable	Not applicable	Not established — no human pharmacokinetic, dosing, or safety data for injected protein exists
General “anti-myostatin”/muscle-growth framing, sold as “Follistatin 344”	Subcutaneous or intramuscular injection	Commonly cited around 100 mcg per day, or several-hundred-microgram doses several times per week	Daily or several times weekly	One to several weeks per cycle	Community / vendor reported, origin unclear; could not be traced to any published clinical or preclinical pharmacokinetic study

Amounts studied in human research

No established or reliably sourced dosing information was identified for injected follistatin-344 protein in human research. The human trials that exist used gene therapy, not protein injection: the Becker muscular dystrophy / sporadic inclusion body myositis trial (NCT01519349) and the Duchenne muscular dystrophy trial (NCT02354781) both delivered rAAV1.CMV.huFollistatin344 as a one-time intramuscular gene-transfer injection at escalating vector-particle dose cohorts, not as repeated doses of the follistatin-344 protein itself. These vector doses are not convertible into an equivalent protein-injection dose, and no such conversion was identified in the literature.

Amounts studied in animal research

Not consistently reported for injected follistatin-344 protein specifically in the sources reviewed for this entry. Related preclinical work on myostatin inhibition (including genetic myostatin knockout and various follistatin-based constructs) exists in the broader muscle-biology literature, but a specific, reliably sourced injectable follistatin-344 animal dosing figure was not identified in this research pass.

Practitioner and community-reported protocols

Peptide vendor sites and community forums circulate injectable protocols for “Follistatin 344,” commonly citing figures in the range of roughly 100 micrograms per day, or several-hundred-microgram doses several times per week, for cycles of one to several weeks. These figures could not be traced to any published clinical or preclinical pharmacokinetic study of injected follistatin-344 protein. Widely repeated online, but its original source could not be verified. Evidence category: community/vendor reported, origin unclear.

Side effects, risks, and limitations

In the published gene-therapy trials, follistatin-344 gene transfer was generally reported as well tolerated at the doses and follow-up durations studied, with safety being the primary endpoint of those small trials; no severe treatment-related toxicities meeting the trials’ predefined stopping criteria were reported in the sources reviewed. However, these safety findings apply to a one-time, localized, AAV-delivered gene-therapy product under close clinical monitoring (including muscle biopsies and MRI follow-up), not to repeated self-administered injections of a manufactured follistatin-344 protein — a route of use for which no comparable human safety data exists. Broader mechanistic concerns about myostatin/activin pathway inhibition include unknown effects on other TGF-beta superfamily-dependent tissues, unknown long-term consequences of sustained pathway blockade, and, as with any unregulated “research peptide,” substantial uncertainty about the purity, potency, correct folding, and actual identity of vendor-sold follistatin-344 products, given that follistatin is a relatively large, structurally complex protein that is more difficult to manufacture correctly than a short synthetic peptide.

Regulatory and developmental status

Follistatin-344, as an injectable protein product, is not an approved drug anywhere and has no identified active clinical development program in that form. Its only meaningful clinical development has been as a gene-therapy transgene: a completed Phase 1 trial in Becker muscular dystrophy and sporadic inclusion body myositis (NCT01519349, Nationwide Children’s Hospital, completed 2017) and a completed Phase 1/2 trial in Duchenne muscular dystrophy (NCT02354781, completed 2017), both using rAAV1.CMV.huFollistatin344 delivered by intramuscular injection. Neither program has been identified as having advanced to a registration-directed pivotal trial or approval. Separately, unrelated early-phase gene-therapy work combining follistatin with other genes (for example, klotho) for aging/frailty indications has been sponsored by other groups, but these also use gene delivery, not protein injection, and are not evidence of progress toward an approved injectable follistatin-344 product. Date verified: 2026-09-20.

Frequently asked questions

What is Follistatin 344?

It is a specific isoform of the naturally occurring human protein follistatin, which binds and blocks myostatin and related growth-limiting signals in muscle.

Is the “Follistatin 344” sold by peptide vendors the same as what was studied in clinical trials?

Not in the way it is used. The clinical trials delivered the follistatin-344 gene via a viral vector so muscle cells produce the protein themselves over time. Vendor products are sold as a manufactured protein meant to be injected directly and repeatedly — a different intervention that has not itself been tested in humans.

Does human data exist for injectable follistatin-344?

No dedicated human pharmacokinetic, dosing, or safety data for directly injected follistatin-344 protein was identified. The human data that exists is from gene-therapy delivery.

What dosage has been studied?

No established or reliably sourced dosing information was identified for injectable follistatin-344 protein. Gene-therapy trials used vector-particle dose escalation, which is not equivalent to a protein injection dose.

What conditions has follistatin-344 gene therapy been tested in?

Becker muscular dystrophy, sporadic inclusion body myositis, and Duchenne muscular dystrophy, each in small, early-phase trials.

Did the gene therapy trials show a benefit?

Some reported increases in muscle fiber size and, in the inclusion body myositis cohort, improved six-minute walk distance, but these were small, largely uncontrolled early-phase studies, and a published critique specifically challenged whether the functional-improvement claims were adequately supported.

Is it approved or regulated as a drug?

No. It is not approved in any form, and injectable protein versions sold online are unregulated research chemicals.

What remains unknown?

Essentially everything about injected follistatin-344 protein specifically: absorption, half-life, an effective or safe dose, and whether it produces any of the effects seen with gene-delivered, muscle-produced follistatin-344.

Bottom line

Follistatin 344 is a real, biologically active protein isoform with a legitimate, if very early-stage, human research record — but that record belongs almost entirely to gene therapy, not to the injectable protein product sold under this name by research-chemical vendors. The gap between “follistatin-344 delivered by AAV gene therapy in a handful of small clinical trials” and “follistatin-344 protein injected repeatedly as a peptide” is substantial, and no human data bridges it. Anyone evaluating claims about injectable follistatin-344 should treat the gene-therapy literature as informative about the underlying biology, not as evidence for the safety, dosing, or effectiveness of the injectable product actually being sold.