Growth hormone axis

Follistatin-344

Also called FS344, FS-344, Follistatin (FST) 344-amino-acid isoform, rAAV1.CMV.huFollistatin344 (gene therapy vector), Follistatin-315, FS315

Follistatin is a natural protein that blocks myostatin, a signal that limits how big muscles grow. It is a full-size protein rather than a short peptide: the 344-amino-acid form (follistatin-344) is trimmed to a 315-amino-acid protein, but it is often discussed alongside peptides, much like growth hormone. In animals, adding extra follistatin made muscles bigger and stronger. In people, it has only been tested as a gene therapy (a virus carrying the follistatin gene injected into leg muscles) in a few small trials in muscle-wasting diseases. It has never been approved as a drug, it is banned in sport, and there is no published human trial of injecting follistatin protein itself.

Early human dataSome small human studies exist; evidence is limited.

New · under editorial review. Added October 29, 2026 by our automated research, with every claim sourced. A person has not checked this page yet; treat it with extra care. How we research

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Illustrative chain. The exact sequence has not been published in a source we could verify.

At a glance

Type: Peptide
Size: 344 amino acids before processing; a 29-amino-acid signal piece is removed, leaving a 315-amino-acid secreted protein (FS315). A shorter splice form gives FS288 [1][3].
Molecular weight: About 38 kDa calculated for the unmodified 344-amino-acid chain (UniProt); the natural protein carries sugar groups, so its real mass differs [3]
Origin: A protein, not a short peptide: product of the human FST gene (UniProt P19883), made by alternative splicing and carrying sugar groups; the full sequence is in UniProt [3]. At over 40 amino acids it counts as a protein under FDA rules [10]. Human trials used a viral vector (AAV1) carrying this gene, not the protein itself [1].
Evidence: Early human data
US FDA status: Not FDA-approved in any form. Gene-therapy versions have been tested only in small investigational trials ¹²⁷.

Overview

How it works

Established (lab and animal studies): follistatin binds myostatin and stops it from attaching to its receptor (activin receptor IIB). In receptor tests it blocked myostatin binding strongly ⁴. It also binds other members of the same family, such as activin, GDF11 and BMPs ³. Because myostatin normally holds back muscle growth, blocking it lets muscles grow larger ⁴. Proposed: the gene-therapy teams picked the FS344 form because it is processed into FS315, which circulates in the blood and binds tissues less, which they hoped would avoid effects on the pituitary and sex-hormone system linked to the shorter FS288 form ¹.

What research shows

Becker muscular dystrophy (human Phase 1/2a gene-therapy trial, n=6 men, no control group): after AAV1-follistatin injections into both thighs, 6-minute walk distance rose by 58, 125 and 108 m in three men, by 29 m in one, and did not clearly improve in two. Knee strength did not consistently increase. Muscle biopsies showed larger fibers and less scarring ¹.
Sporadic inclusion body myositis (human Phase 1 gene-therapy trial, n=6 treated vs 8 matched untreated, not randomized): walk distance improved by a median of 56 m per year in treated patients vs a decline of 25.8 m per year in untreated patients. Four of six improved by 58–153 m; two barely changed. Patients also followed an exercise program and took prednisone, which could have affected results ².
Duchenne muscular dystrophy (human Phase 1/2 gene-therapy trial, n=3, completed 2017): results posted on ClinicalTrials.gov report no dose-limiting toxicities; at 2 years none of the 3 had a longer 6-minute walk and 1 improved on a motor-function scale, and planned follow-up muscle biopsies were cancelled because of disease progression. No journal publication was found ⁷.
Mice (animal study): one AAV1-follistatin-344 injection increased muscle mass and grip strength in normal mice and in mdx mice (a Duchenne model), with effects lasting up to about 2 years and less muscle scarring in older mdx mice ⁵.
Monkeys (animal study, n=6 cynomolgus macaques): a single gene-therapy injection into one thigh muscle was reported to increase the size and strength of that muscle. The journal published a correction to this paper in August 2026 that we could not read, so its specific figures are not repeated here ⁶¹³.
Mice engineered to make extra follistatin (animal study): in the strongest line, muscles weighed 194–327% more than normal, from both more and larger muscle fibers ⁴.

Doses studied in people

Study or label	Dose used	Source
Phase 1/2a gene-therapy trial in Becker muscular dystrophy (n=6); gene-therapy vector dose, not a follistatin protein dose	AAV1.CMV.FS344 gene-therapy vector, 3 × 10^11 or 6 × 10^11 vector genomes/kg per leg, one-time intramuscular injections into both quadriceps (a dose of virus particles carrying the gene, not of follistatin protein)	¹
Phase 1 gene-therapy trial in sporadic inclusion body myositis (n=6); gene-therapy vector dose, not a follistatin protein dose	rAAV1.CMV.huFS344 gene-therapy vector, 6 × 10^11 vector genomes/kg, one-time injections into the quadriceps of both legs (a dose of virus particles carrying the gene, not of follistatin protein)	²
Phase 1/2 gene-therapy trial in Duchenne muscular dystrophy (n=3; dose from ClinicalTrials.gov posted results, no journal publication); gene-therapy vector dose, not a follistatin protein dose	rAAV1.CMV.huFollistatin344 gene-therapy vector, single total dose of 2.4 × 10^12 vector genomes/kg (1.2 × 10^12 per limb) into leg muscles (a dose of virus particles carrying the gene, not of follistatin protein)	⁷

The only human doses are one-time gene-therapy vector doses (virus particles per kg of body weight) given in research hospitals; they cannot be converted into an amount of follistatin protein. No human trial has tested injected follistatin protein, and amounts circulating online are not based on human studies.

Read this first: these are the amounts given to participants in published studies or written on official prescribing labels, reported for reference. They are not recommendations. Trial doses were chosen for specific patients under medical supervision, and a dose that is safe in one person can be harmful in another.

Vial math

Many research vials hold 5 or 10 mg of powder. This table shows how much ends up in each mark on a U-100 insulin syringe for common amounts of added liquid. It is arithmetic only, not a dose.

Vial	Liquid added	Concentration	Per unit (0.01 mL)
5 mg	1 mL	5 mg/mL	50 mcg	Open in calculator
5 mg	2 mL	2.5 mg/mL	25 mcg	Open in calculator
5 mg	3 mL	1.67 mg/mL	16.7 mcg	Open in calculator
10 mg	1 mL	10 mg/mL	100 mcg	Open in calculator
10 mg	2 mL	5 mg/mL	50 mcg	Open in calculator
10 mg	3 mL	3.33 mg/mL	33.3 mcg	Open in calculator

More sizes: all vial sizes · How this works: Syringes and units · Reconstitution guide.

Safety

Gene-therapy trials (n=6 each): no treatment-related serious side effects were reported; liver, kidney, blood and sex-hormone tests (FSH, LH, testosterone, estrogen) stayed normal ¹².
Immune reactions: in the Becker trial, some patients showed T-cell responses to follistatin or to the virus shell, with no consistent pattern; patients were given prednisone around the time of injection to limit immune reactions to the vector ¹².
Duchenne trial (n=3): posted results list one serious adverse event (head injury from a fall) and no dose-limiting toxicities ⁷.
Follistatin also binds activin and other growth factors, not just myostatin ³. Its shorter form is involved in reproductive hormone control, which is why trial teams watched hormone levels ¹.
Long-term safety in humans is unknown. Products sold online as 'follistatin' are not FDA-approved and have not been tested in human trials; in 2020 FDA cited a compounding pharmacy that had made follistatin products ⁹.

Report side effects of any drug or product to the FDA through MedWatch. For a possible poisoning in the US, call Poison Control at 1-800-222-1222.

Legal status

FDA: Not FDA-approved in any form. Gene-therapy versions have been tested only in small investigational trials ¹²⁷.
Compounding: Not eligible for 503A compounding: in an April 2020 warning letter, FDA said follistatin was not the subject of a USP/NF monograph, was not a component of an approved drug, was not on the 503A bulks list and had not been nominated for it ⁹. Follistatin does not appear in FDA's list of nominated 503A bulk substances under evaluation (categories updated May 14, 2026) ¹⁴. At more than 40 amino acids it also meets FDA's regulatory definition of a protein ¹⁰.
Sports (WADA): Prohibited at all times. The WADA 2026 Prohibited List names follistatin as an example of a myostatin-binding protein under S4.3 (agents preventing activin receptor IIB activation) ¹¹¹²¹⁵. Gene transfer methods are separately banned under M3 (gene and cell doping) ¹¹.
Other: No approval found in any country in the sources reviewed.

Rules change often. See Legal status of peptides for how these categories work.

History

In 2001, Lee and McPherron showed that follistatin binds myostatin and that mice engineered to make extra follistatin had much larger muscles ⁴. In 2008, the Kaspar and Mendell group reported that a single gene-therapy injection of follistatin-344 boosted muscle size and strength in mice for over 2 years ⁵. In 2009, the same team reported larger, stronger thigh muscles in monkeys ⁶. Human gene-therapy trials at Nationwide Children's Hospital followed: Becker muscular dystrophy (published 2015), sporadic inclusion body myositis (published 2017), and a small Duchenne trial (3 participants, completed 2017) ¹²⁷⁸.

Common questions

Is follistatin-344 FDA-approved?

No. It is not approved in any form; gene-therapy versions have only been tested in small research trials ¹²⁷.

Has follistatin been tested in humans?

Only as a gene therapy injected into leg muscles, in small trials of 3 to 6 patients with Becker or Duchenne muscular dystrophy or inclusion body myositis ¹²⁷. Some patients walked farther, but there were no placebo groups, so the results are early ¹².

Is follistatin a peptide?

Strictly, no: it is a protein of 315 amino acids after processing ³, and FDA rules call any chain of more than 40 amino acids a protein ¹⁰. This site covers it with peptide drugs, as it does for growth hormone, because it is discussed and sold alongside them.

Is follistatin banned in sport?

Yes. WADA's 2026 list names follistatin as a banned myostatin-binding protein at all times, and gene doping is banned too ¹¹¹⁵.

What are the known risks?

The small gene-therapy trials reported no treatment-related serious side effects and normal hormone tests ¹². Long-term human safety is unknown, and products sold online have not been tested in human trials ⁹.

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Last reviewed 2026-10-29 · How we research · Found an error? See our corrections policy.