A natural protein that blocks myostatin, the brake on muscle growth. Human data come only from small gene-therapy trials, not injected protein.
Also called Follistatin 344, FS344, FST-344, FS315, Follistatin-315, Follistatin, FST, rAAV1.CMV.huFollistatin344, AAV1-FS344
When Follistatin-344's status or evidence changes, it's in the next morning's email.
Follistatin is a natural protein that binds and blocks myostatin, activin and related signals. Myostatin limits muscle growth, so blocking it lets muscle grow. FS344 is one of two forms the follistatin gene makes: a 344-amino-acid precursor that loses a 29-amino-acid signal sequence to become FS315, the form that circulates in blood. The other form, FS288, stays bound to cell surfaces and blocks activin more strongly, which is why researchers chose FS344 to limit effects on the pituitary's FSH output. Mice engineered to make extra follistatin in muscle grew muscle on a scale comparable to myostatin knockout mice.
The human data are from gene therapy, not from injecting the protein. At Nationwide Children's Hospital, doctors injected a virus (AAV1) carrying the FS344 gene into the thigh muscles of 6 men with Becker muscular dystrophy and 6 people with sporadic inclusion body myositis. In the Becker trial, 4 of 6 walked 29 to 125 meters farther in 6 minutes and 2 did not improve. In the myositis trial, treated patients gained 56 meters a year while 8 untreated comparison patients lost 25.8; a critic argued in the same journal that the comparison did not support the claim. No serious side effects were reported. In 2022 and 2023 a startup, Minicircle, ran an unpublished plasmid follistatin trial in 43 healthy people on Roatán, Honduras, outside FDA oversight; according to news reports, it took place in the Próspera special economic zone.
Vials sold online as 'Follistatin-344' are a different matter. They are research-chemical protein meant to be injected, and no study has measured what happens when follistatin protein is injected into a person: how long it lasts, whether it reaches muscle, or whether it is safe. The labels often name the precursor form, which includes a signal sequence the body cuts off. No follistatin product is approved anywhere, and WADA names follistatin under S4.3 as a myostatin inhibitor banned at all times.
What it binds to, and what happens inside the body after that, step by step.
Follistatin wraps around TGF-beta family signals such as myostatin, activin and GDF11 in the space outside cells, covering the spot they use to bind their type II receptors. With myostatin blocked from activin receptor IIB, the normal brake on muscle fiber growth is released. Because follistatin also blocks activin, which drives FSH release from the pituitary, the FS288 form was avoided in trials in favor of FS344, which has about 10-fold lower activin affinity.
Human results and animal results are kept apart below. A result in rats tells you a mechanism is plausible. It does not tell you what happens in a person.
The C grade rests entirely on gene therapy: two open-label trials of AAV1.FS344 injected into thigh muscle, 6 patients each, plus a 3-patient Duchenne safety study. Walking distance improved in most treated patients, but there was no placebo group, and the myositis result was publicly disputed. For follistatin protein injected under the skin, as research vials are used, there are no human studies at all, so that use sits at grade D.
6 men with Becker muscular dystrophy, AAV1.CMV.FS344 injected into both quadriceps
Four of 6 improved their 6-minute walk by 29 to 125 meters; 2 did not change. No adverse effects; biopsies showed less fibrosis and larger fibers.
Read the study6 people with sporadic inclusion body myositis, 6 x 10^11 vg/kg per leg, with an exercise program; 8 untreated matched comparisons
6-minute walk distance rose 56.0 m/year in treated patients vs a 25.8 m/year decline in untreated (p = 0.01); 4 of 6 gained 58 to 153 m.
Read the study3 boys with Duchenne muscular dystrophy, 2.4 x 10^12 vg/kg total
Posted results report 0 dose-limiting toxicities; the only serious adverse event was a head injury from a fall.
Read the studyCynomolgus macaques, AAV1-FS344 injected into the quadriceps
Pronounced, lasting gains in muscle size and strength with no abnormal changes in key organs.
Read the studyNormal and dystrophic mice, single AAV gene dose
Muscle mass and strength stayed higher for more than 2 years, even when given to aged mice.
Read the studyTransgenic mice expressing follistatin in skeletal muscle
Follistatin blocked myostatin binding to activin receptor IIB, and the mice showed muscle mass increases comparable to myostatin knockouts.
Read the studyWhen a new Follistatin-344 study in people is published, the daily email covers it.
These are questions the published research has not answered.
No product containing Follistatin-344 is approved as a medicine by the FDA.
No follistatin product, protein or gene therapy, is approved anywhere. The academic AAV1.FS344 trials at Nationwide Children's Hospital (NCT01519349, NCT02354781) were completed by 2017. In 2026 a company called Unlimited Biotechnology was recruiting 12 people in Honduras for an AAV9-follistatin plus VEGF plasmid trial in age-related muscle loss (NCT07443826), and Minicircle completed a 14-person klotho and follistatin plasmid study in April 2026 (NCT07285629); neither has posted results. WADA S4.3 names 'myostatin-binding proteins (e.g. follistatin, myostatin propeptide)', and M3 separately bans gene transfer, both at all times.
When the FDA or WADA changes Follistatin-344's status, it's in the next morning's email.
No human half-life has been published for Follistatin-344, so we do not draw a clearance curve. Half-lives quoted online for it are usually estimates from animal data.
This table records the doses that trials and approved labels used, with the source for each row. It is a record of research, not a recommendation, and it is open to free members.
Which symptoms are expected and which mean stop and get help: side effects and red flags.
Tap a claim to see what the studies found.
The human trials put a gene into thigh muscle so the muscle itself made follistatin over time. No published study has injected follistatin protein into a person.
Two open-label trials of 6 patients each reported walking gains, and biopsies showed less fibrosis. There was no placebo group, 2 of 6 Becker patients did not improve, and a neurologist disputed the myositis claims in the same journal.
FS344 is the 344-amino-acid precursor; the body removes a 29-amino-acid signal sequence to make the 315-residue circulating protein. FS288 is the separate cell-surface form, which binds activin about 10 times more strongly.
Follistatin does not block only myostatin. It also binds activin, GDF11 and bone morphogenetic proteins, and the long-term consequences of that in adults have not been studied.
Research-grade recombinant follistatin is shipped freeze-dried. There are no published stability data for injection use.
No published stability data for any solution intended for injection.
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