Follistatin-344.
Follistatin-344 (FS-344) is an alternatively spliced isoform of human follistatin, a naturally occurring secreted glycoprotein that acts as a high-affinity antagonist of several TGF-beta superfamily ligands.
Follistatin-344 (FS-344) is an alternatively spliced isoform of human follistatin, a naturally occurring secreted glycoprotein that acts as a high-affinity antagonist of several TGF-beta superfamily ligands. Follistatin-344 is research / preclinical, and PepCue grades its published evidence F tier (28/100). This is a research reference, not medical or dosing advice.
What it is
Follistatin-344 (FS-344) is an alternatively spliced isoform of human follistatin, a naturally occurring secreted glycoprotein that acts as a high-affinity antagonist of several TGF-beta superfamily ligands. The "344" refers to a 344-amino-acid precursor variant; relative to the longer FS-315 serum isoform, it lacks the C-terminal acidic tail and was selected for therapeutic use partly to reduce off-target heparin/cell-surface binding. In gene-therapy programs it is the FS344 transgene that is delivered, not an injected peptide product, although it is now marketed in gray-market channels as a "research peptide."
How it works
Follistatin works by binding and neutralizing myostatin (GDF-8) and related ligands such as activin A, GDF-11, and several BMPs, preventing them from engaging activin type II receptors. Because myostatin is a dominant negative regulator of skeletal muscle mass, removing this brake promotes satellite-cell activation, myofiber hypertrophy, and reduced fibrosis. Critically, follistatin neutralizes a broader set of ligands than myostatin-only blockade, which is why follistatin overexpression produces larger muscle gains in animals than myostatin knockout alone. The foundational biology traces to McPherron, Lawler and Lee (Nature, 1997), who showed myostatin loss roughly doubles muscle mass in mice.
Mechanism pathways
Removing the biological brake that limits skeletal muscle growth.
Skeletal muscle mass is actively restrained, not merely permitted. Myostatin, also called GDF-8, is a member of the TGF-beta superfamily secreted by muscle itself, and it signals through activin type II receptors, principally ActRIIB, partnered with a type I receptor. Activation phosphorylates SMAD2 and SMAD3, which enter the nucleus and suppress the transcriptional programme for muscle growth while promoting protein degradation pathways. The biology is unusually well demonstrated: naturally occurring loss-of-function mutations in myostatin produce dramatic muscle overgrowth in cattle, dogs, mice, and at least one documented human case. Few pathways have that quality of natural experiment behind them. Removing the brake therefore promotes satellite cell activation, myofibre hypertrophy, and reduced fibrosis. Two strategies are represented here. One uses a soluble decoy receptor: the extracellular ligand-binding portion of ActRIIB fused to an antibody fragment, which circulates and captures ligands before they can reach receptors on muscle. The other uses a naturally occurring binding protein that neutralises myostatin along with activin A, GDF-11, and several bone morphogenetic proteins, preventing them from engaging the type II receptors at all. The crucial difference between them is breadth, and breadth is a double-edged property. Neutralising more ligands produces larger effects on muscle than blocking myostatin alone, which is why broader inhibition is attractive. But ActRIIB and the related receptors serve ligands with jobs elsewhere in the body, including in blood vessels, bone, and reproductive tissue. Capturing those ligands produces effects well outside muscle, and this is not a theoretical concern: clinical development of the decoy receptor approach for muscle-wasting conditions was halted after non-muscle effects including nosebleeds and gum bleeding emerged in human studies, attributed to interference with vascular-related ligands. Honest position: the pathway is real and important, and it remains a legitimate target under active pharmaceutical development. What has not been achieved is a clean separation between the muscle benefit and the off-target consequences of blocking a receptor family with broad physiological responsibilities. No agent in this group is approved for any indication. The gene-therapy-adjacent versions of this approach raise entirely separate and more serious issues. Material sold under these names is unregulated, and for the binding-protein approach in particular, whether an injected peptide fragment reproduces the biology of the full protein at all is uncertain.
The evidence
Human evidence is limited to two small, open-label AAV1-delivered FS344 gene-therapy trials from Nationwide Children's Hospital (Mendell and colleagues), not to any injected-peptide product. A Phase 1/2a trial in Becker muscular dystrophy (6 subjects; Mol Ther 2015, PMID 25322757) reported six-minute-walk gains in some treated patients (e.g., +58 m and +125 m in two subjects) with histological evidence of reduced fibrosis and fiber hypertrophy. A companion sporadic inclusion body myositis trial (6 subjects; Mol Ther 2017, PMID 28279643) reported improved annualized six-minute-walk distance versus untreated controls, though responses were heterogeneous and the comparison used a non-randomized matched control group. These are early-phase, unblinded, very small studies; large-animal support comes from a nonhuman-primate follistatin gene-delivery study (Kota et al., Sci Transl Med 2009, PMID 20368179). No randomized controlled trial, and no trial of FS-344 as a standalone injectable peptide, has demonstrated efficacy. The sIBM functional claims also drew a published methodological critique in Molecular Therapy.
The evidence, in brief
A myostatin/activin antagonist studied mainly in preclinical and gene-therapy contexts for muscle-wasting disease. An early phase-1/2a AAV1-FS344 trial in Becker muscular dystrophy reported functional changes in a small safety/feasibility setting. Experimental, not approved.
- A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophyMol Ther, 2015 (PMID 25322757)
Evidence maturity
An evidence-only reading: approval status, human vs preclinical data, mechanism and safety. Popularity never raises it. Research file #019
Mostly preclinical or mechanistic; little human data.
Findings come mainly from animal models, not people.
Claim receipts
Popular claims about Follistatin-344, checked against the state of the evidence. The verdict describes evidence maturity, never an invented study result.
All human data come from AAV-delivered gene therapy, not from an injected peptide product.
Two very small open-label trials reported walking gains, but they were unblinded and drew a published methodological critique.
Broader TGF-beta ligand antagonism is a mechanistic and preclinical argument, not a demonstrated human advantage.
Follistatin was first identified as an inhibitor of FSH secretion, and reproductive and pituitary-gonadal effects remain unresolved in humans.
Safety profile
In the two small gene-therapy trials, intramuscular AAV1.FS344 was reported as generally well tolerated over follow-up exceeding two years, but these cohorts are far too small to characterize real risk. Because follistatin broadly inhibits TGF-beta/activin signaling, theoretical and preclinical concerns include effects on reproductive tissues (follistatin was first identified as an inhibitor of FSH secretion), the pituitary-gonadal axis, vascular and cardiac remodeling, and possible influence on tumor biology, none of which are adequately resolved in humans. Gray-market "follistatin-344 peptide" products carry the additional, unquantified hazards of unverified identity, purity, sterility, and the fundamental mismatch that human data come from a delivered gene, not an injected protein. There is no established human safety profile for self-administered FS-344.
Compound notes
- Follistatin-344 is an isoform of follistatin, a protein that binds and inhibits myostatin (a negative regulator of muscle growth).
- Inhibiting myostatin promotes muscle growth in animal and gene-therapy models.
- Not FDA-approved; research-only, often in a gene-therapy context.
- Powerful growth modulation carries unknown long-term human risks.
Regulatory status
Follistatin-344 is not approved by the FDA (or any major regulator) for any indication; it has only been studied investigationally as an AAV-delivered gene therapy and is sold elsewhere strictly as a research-use-only chemical, not a medicine. Myostatin-pathway inhibition is also of interest to anti-doping bodies, and follistatin/myostatin inhibitors fall under WADA's prohibited categories.
Sold research-use-only; human evidence is limited or preclinical.
By the numbers
- 01FS-344 is a 344-amino-acid alternatively spliced isoform of human follistatin, selected to limit off-target cell-surface binding versus the longer FS-315 isoform
- 02It antagonizes multiple TGF-beta ligands (myostatin/GDF-8, activin A, GDF-11), giving broader muscle-growth effects than myostatin-only blockade
- 03All human data come from AAV1-delivered FS344 gene therapy in two ~6-patient trials (Becker muscular dystrophy and sporadic inclusion body myositis), not from an injected peptide
- 04The Becker muscular dystrophy trial was described as among the first gene therapies to show functional improvement in a muscular dystrophy, but it was tiny and unblinded
- 05No randomized controlled trial supports efficacy, and the sIBM functional claims received a published methodological critique
- 06Not FDA-approved for any use; sold only as a research chemical and relevant to WADA anti-doping rules
Follistatin-344: research formats
Choose the format you are researching to see route-specific notes.
All human data come from AAV gene therapy delivering the FS344 transgene, not from an injected peptide.
Follistatin-344 is best understood as a transgene, not an injectable product. The only human data come from two very small AAV1-delivered FS344 gene-therapy trials, in Becker muscular dystrophy and in sporadic inclusion body myositis. Neither studied an injected follistatin protein, so nothing in the literature establishes a route, a concentration, or a schedule for the material sold as a research peptide.
Gene therapy and peptide injection are fundamentally different delivery methods. Outcomes reported from AAV-delivered FS344 say nothing about what happens when a lyophilized peptide product is reconstituted and injected.
Follistatin-344 is not approved for any indication, and myostatin-pathway inhibitors fall under WADA's prohibited categories.
Sources
Every factual claim above resolves to a real, published source.
- A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophyMolecular Therapy, 2015, PMID 25322757 (Mendell JR et al.)
- Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional OutcomesMolecular Therapy, 2017, PMID 28279643 (Mendell JR et al.)
- Follistatin gene delivery enhances muscle growth and strength in nonhuman primatesScience Translational Medicine, 2009, PMID 20368179 (Kota J et al.)
- Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member (myostatin)Nature, 1997, PMID 9139826 (McPherron AC, Lawler AM, Lee SJ)
Cite this page
PepCue. “Follistatin-344: the evidence.” PepCue, reviewed June 1, 2026. https://www.pepcue.app/p/follistatin-344.
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