what is the difference
Follistatin-344 and follistatin gene therapy
One name, two entirely different interventions: a protein sold in a vial, and a virus carrying a gene into a named muscle in a hospital. The published human results belong to the second.
Follistatin is a protein the body makes that binds myostatin and several of its relatives. Myostatin is the brake on muscle growth: cattle born with a broken myostatin gene grow visibly larger muscles, which is the observation the whole field started from [4]. FS-344 is one of the two main forms of follistatin, and at 344 amino acids it is a glycoprotein rather than a peptide, far above the forty amino acid line United States drug law draws between the two.
Follistatin gene therapy is a different intervention that shares the word. In the published trials, an adeno-associated virus carrying the follistatin gene was delivered directly to the quadriceps muscles of both legs, once, in a hospital, so that the muscle itself produces the protein [2] [3]. A single delivery of a gene into a named muscle and a protein drawn from a vial have unrelated pharmacokinetics, and the human results that circulate for the injectable were produced by the first.
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This page reports what has been documented about a compound. It is not medical advice, not a diagnosis, not a dosing protocol, and not a recommendation to obtain or use anything described here. Talk to a licensed clinician about anything concerning your health. Read the full disclaimer.
Reviewed against editorial standards Updated
The short answer
Follistatin-344 is a 344 amino acid glycoprotein sold as an injectable. Follistatin gene therapy is an experimental product in which a virus carrying the follistatin gene is delivered into muscle tissue on one occasion under hospital conditions. The six-minute walk results people quote belong to the gene therapy [2] [3]. The one published human record of the injected protein is a harm report: a retrospective series of eleven bodybuilding athletes who developed central serous chorioretinopathy, a condition in which fluid collects under the retina and vision drops, after high-dose injections [1]. On 2 August 2026 no registered trial delivered a recombinant follistatin-344 protein by injection [5].
Side by side
| Property | Follistatin-344 protein | Follistatin gene therapy |
|---|---|---|
| What it is | A 344 amino acid glycoprotein, one of the two main forms of natural follistatin. Not a peptide by any working definition | A viral or plasmid vector carrying the follistatin gene, so that muscle cells produce the protein themselves. In the published trials the construct is rAAV1.CMV.huFS344 [2] [3] |
| How it reaches the body | As a protein, from a vial, repeatedly | As a gene, delivered on one occasion directly to the quadriceps muscles of both legs, in a hospital, so the muscle itself produces the protein [2] [3] |
| Published human results | One report, and it is a harm report. Eleven male bodybuilding athletes, mean age 36.8 years, developed central serous chorioretinopathy after high-dose injections [1] | Two trials. A phase 1/2a in Becker muscular dystrophy, six patients, six-minute walk gains of 58, 125, 108 and 29 metres in four of them and no change in two [3]. A trial in sporadic inclusion body myositis, six participants with an exercise regimen attached, annualising to plus 56.0 metres against minus 25.8 metres in eight matched untreated comparators [2] |
| Design of that human work | Retrospective case series from a single hospital, no control arm, no protocol, patients presenting after the fact [1] | Small open-label trials with matched untreated comparators rather than randomisation [2] [3] |
| What the registry holds | 0 registrations deliver a recombinant follistatin-344 protein by injection [5] | 78 records on a term search, 13 of them observational by the registry's own study-type field. Six name follistatin in the intervention field, four of those delivering it as a virus or a plasmid. Most of the rest give something else and measure follistatin in blood [5] |
| Approval status | Not an approved drug or biological product in any country | Experimental. No follistatin gene therapy is approved either |
| Evidence grade on this site | Not graded. A grade is derived from tiered claim rows, and that source review has not been run here | Not graded. The two published trials are cited directly instead |
| Anti-doping position | Follistatin acts on myostatin, and agents modifying myostatin function fall within the World Anti-Doping Agency Prohibited List, whose 2026 edition took effect on 1 January 2026 [6] | Not sold and not obtained by an individual. The published deliveries took place inside registered trials [2] [3] [5] |
| What is missing | Any study of any design measuring muscle growth in a person given the injected protein | A randomized controlled trial. The published work is small and open label |
Why the two get mixed up
The construct in the published trials is named after the protein. Its full designation, rAAV1.CMV.huFS344, contains FS344, so a search for follistatin-344 returns the gene therapy papers with the vendor's product name apparently in the title [2] [3]. Two things sharing a string is enough for a marketing page, and once a walking-test figure has been lifted out of a gene therapy trial it travels without the delivery method attached.
The registry makes it worse rather than better. A term search on follistatin returned 78 records on 2 August 2026, and that number gets quoted as though it were a trial programme. Thirteen are observational by the registry's own study-type field, and only six name follistatin in the intervention field at all, four of them delivering it as a virus or a plasmid [5]. Most of the remainder give a person something else entirely, an exercise protocol, a diet, a statin, and measure follistatin in blood as an outcome. Counting records that contain a word is not counting trials of a product. Underneath both problems sits a category error: 344 amino acids makes this a glycoprotein rather than a peptide, and getting a protein that size to survive and reach muscle is a different problem from the one a short peptide poses.
What the published record covers for each
The human work on gene transfer is real, small, and open label. In a phase 1/2a trial in Becker muscular dystrophy, an adeno-associated virus carrying the FS344 sequence was delivered to the quadriceps muscles of both legs in six patients across two dose cohorts, with the six-minute walk test as the primary outcome. Two patients in the first cohort gained 58 and 125 metres and one showed no change; in the second cohort two gained 108 and 29 metres and one showed no improvement. Histology showed reduced fibrosis and larger fibre size [Human open-label] [3]. In a later trial in sporadic inclusion body myositis, the same construct was delivered to the quadriceps of both legs in six participants, with an exercise regimen attached; walking performance annualised to a median one-year change of plus 56.0 metres against minus 25.8 metres in eight untreated comparators matched for age, sex and baseline [Human open-label] [2]. Neither trial was randomized, both are small, and both delivered a gene rather than a protein.
The one published human record of the injected protein runs the other way. A retrospective case series from a single hospital described eleven male bodybuilding athletes, mean age 36.8 years, who presented with reduced visual acuity and imaging consistent with central serous chorioretinopathy after high-dose follistatin-344 injections. All had injected a complete vial. Eight had a history of one previous injection and three of several; ten had findings in one eye and one in both. In the eight with a single previous injection the subretinal fluid cleared over an average of 2.3 months, and recurrence occurred in all three with a history of repeated injections [Human open-label] [1]. The authors put it as a risk factor to ask about when taking a history rather than as a demonstrated cause.
Behind both sits the biology, and it is a claim about cattle. Two breeds of double-muscled cattle were found to carry mutations in the myostatin gene, establishing that removing the myostatin signal produces visibly larger muscles in an animal [Animal] [4]. What this evidence can and cannot show: that result comes from a genetic difference present from conception in cattle, not from anything given to an adult. It is a reason to investigate the pathway, not a finding about what happens when a protein that binds myostatin is put into a person.
A 2026 narrative review of peptides marketed direct to patients places FS-344 among the unapproved compounds where animal-model results exist and rigorous human data are scarce [7]. That is the position: the walking-test numbers belong to a gene therapy, the harm report belongs to the injected protein, and no study of any design has measured muscle growth in a person given the protein.
Our takeA gene therapy's walking-test result, quoted for a protein injection, alongside a published case series of vision loss in exactly the population that buys it. The two records have one word in common and nothing else.
Frequently asked questions
Is follistatin-344 a peptide?
No. It is a 344 amino acid glycoprotein. United States drug law draws the line between a peptide and a protein at forty amino acids, and this sits far above it. It appears on peptide menus because that is where it is sold, not because the chemistry fits.
Do the human muscle results apply to the injectable?
They belong to a different product. In both published trials, an adeno-associated virus carrying the FS344 sequence was delivered to the quadriceps muscles of both legs on a single occasion, in a hospital [2] [3]. A one-off gene delivery into a named muscle and a protein drawn repeatedly from a vial produce nothing like the same exposure, so this site records the gene therapy results as the gene therapy's and gives the injected protein no tier from them.
How many trials has follistatin been in?
It depends entirely on what is being counted. A ClinicalTrials.gov term search returned 78 records on 2 August 2026. Thirteen are observational by the registry's own study-type field, and only six name follistatin in the intervention field at all, four of which deliver it as a virus or a plasmid [5]. Most of the rest give a person something else, exercise or a diet or a drug, and measure follistatin in blood as an outcome. No registration delivers a recombinant follistatin-344 protein by injection.
What is documented about harm from the injected protein?
One published series. Eleven male bodybuilding athletes, mean age 36.8 years, presented at a single hospital with reduced visual acuity and imaging consistent with central serous chorioretinopathy after high-dose injections, all having injected a complete vial. Ten had findings in one eye and one in both. In the eight with a single previous injection, the subretinal fluid cleared over an average of 2.3 months; recurrence occurred in all three with a history of repeated injections [1]. The authors framed it as a risk factor worth asking about rather than a demonstrated cause.
Where does this stand in sport?
Follistatin acts on myostatin, and agents modifying myostatin function fall within the World Anti-Doping Agency Prohibited List, whose 2026 edition took effect on 1 January 2026 [6]. The gene therapy is a separate matter for a separate reason: it is not sold and not obtained by an individual, and its published deliveries took place inside registered trials [2] [3] [5].
References
- Dağ U, Çağlayan M, Öncül H, Alakuş MF. Central serous chorioretinopathy associated with high-dose follistatin-344: a retrospective case series. Int Ophthalmol. 2020. PMID 32671599 DOI 10.1007/s10792-020-01501-6
- Mendell JR, Sahenk Z, Al-Zaidy S, Rodino-Klapac LR, Lowes LP, Alfano LN, Berry K, Miller N, Yalvac M, Dvorchik I, Moore-Clingenpeel M, Flanigan KM, Church K, Shontz K, Curry C, Lewis S, McColly M, Hogan MJ, Kaspar BK. Follistatin Gene Therapy for Sporadic Inclusion Body Myositis Improves Functional Outcomes. Mol Ther. 2017. PMID 28279643 DOI 10.1016/j.ymthe.2017.02.015
- Mendell JR, Sahenk Z, Malik V, Gomez AM, Flanigan KM, Lowes LP, Alfano LN, Berry K, Meadows E, Lewis S, Braun L, Shontz K, Rouhana M, Clark KR, Rosales XQ, Al-Zaidy S, Govoni A, Rodino-Klapac LR, Hogan MJ, Kaspar BK. A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy. Mol Ther. 2015. PMID 25322757 DOI 10.1038/mt.2014.200
- McPherron AC, Lee SJ. Double muscling in cattle due to mutations in the myostatin gene. Proc Natl Acad Sci U S A. 1997. PMID 9356471 DOI 10.1073/pnas.94.23.12457
- US National Library of Medicine, ClinicalTrials.gov. Registry searches run through API v2 on 2 August 2026. A term search for follistatin returned 78 records, of which 13 carry study type observational. Six name follistatin in the intervention field, and four of those deliver it as a viral vector or a plasmid. An intervention search returned 14 records, and none of them delivers a recombinant follistatin-344 protein by injection. ClinicalTrials.gov. 2026. Registry search
- World Anti-Doping Agency. The 2026 Prohibited List, international standard, effective 1 January 2026. WADA. 2026. Prohibited List
- Mendias CL, Awan TM. Safety and Efficacy of Approved and Unapproved Peptide Therapies for Musculoskeletal Injuries and Athletic Performance. Sports Med. 2026. PMID 41966639 DOI 10.1007/s40279-026-02437-0