Follistatin 344

What Follistatin 344 is

Follistatin 344 has been tested almost only in animals: mice, monkeys and pigs. One small early human trial has been finished, in adults with a muscle disease. As of 2026, the FDA, the EMA and the MHRA have not approved any product based on it.

Follistatin is a protein the body makes and releases from cells. Scientists first purified it from fluid in the ovary. In 1990 it was shown to bind another protein called activin [1]. One gene makes several versions of follistatin. Follistatin 344, or FS344, is named for its length of 344 amino acids, the building blocks of protein. FS344 and a shorter version, FS288, are the two main forms studied in research [6].

Scientists study FS344 because it blocks two related growth signals. Both belong to a family called TGF-beta. The first is myostatin, also called GDF-8. It was found in 1997 and is the main brake on muscle size [2]. The second is activin, which has its own line of research in muscle loss [6].

In 2001, mice bred to make extra follistatin grew much more muscle. The extra follistatin overcame the limit that myostatin sets. That result made follistatin a tool for studying the myostatin and activin pathway. It did not make it a treatment [3].

FS344 is unusual among the compounds on this site. Scientists almost never inject it as a protein or a peptide. They deliver the gene for it. The DNA is packed into a virus used as a carrier, most often one called AAV1. After an injection, the animal’s own muscle makes the follistatin protein and keeps making it [16]. Nearly all the published FS344 work was done this way, and that shapes how the results should be read.

This guide is for research and education only.

How much research there is

Many animal studies, by gene delivery, and one small human trial.

Question	Answer
How far the research has gone	Animal studies in rodents, monkeys and pigs. One finished early human gene therapy trial (phase 1/2a) [18]
Kinds of study	Animals bred to carry the gene, and gene delivery by AAV1 virus. One small human trial with no blinding
What was tested	Normal mice and mdx mice, which model muscular dystrophy. Cynomolgus macaques. Pigs. Adults with Becker muscular dystrophy, in a trial only
Checked by other labs	The basic idea of blocking myostatin and activin has been confirmed by many separate labs in several species. The FS344 results were seen in rodents and monkeys by gene delivery, and in pigs bred to carry the gene. The gene delivery groups overlap
How it was given	Mostly as a gene carried by AAV1. This is very different from how a normal peptide or protein moves through the body, and it matters when reading the results
Longest follow-up	Several months in the human Becker trial group [19]. Some animal studies watched for longer
Approval	Experimental. No FDA, EMA or MHRA approval for any FS344 product
Do the results agree	Yes in animals: more muscle and more strength in rodents, monkeys and pigs. Human data come from one small early group

How it might work

Myostatin and activin both hold muscle growth back. They act through receptor groups on the cell surface that overlap. Follistatin binds both signals and shuts them off. Antibodies that block only myostatin have built less muscle in animals than follistatin has. Papers suggest the reason is that FS344 blocks both signals at once [21].

Blocking myostatin

Mice that lack the myostatin gene grow far more muscle than normal mice. That 1997 finding showed myostatin is the body’s main natural limit on muscle growth [2].

Later work showed follistatin sticks directly to myostatin. The pair forms a complex. In cells and animals, that stopped myostatin from blocking the growth of new muscle cells [7].

In the 2001 study, mice with extra follistatin gained even more muscle than mice with no myostatin at all. The authors suggested follistatin must also block something else that limits muscle. Later research showed they were right [3].

Blocking activin

Myostatin and activin A use the same set of receptors on the cell surface. The main one is activin receptor type IIB, or ActRIIB. Some of the largest muscle gains in animal research came from decoy receptors. These float free and block the shared receptor, not one signal alone [9]. Follistatin binds activin as well as myostatin. So it blocks about as much of the pathway as a decoy does, and no separate receptor-targeted tool is needed [9].

A 2018 study tested this head to head. Scientists built an engineered follistatin that blocks both signals. They compared it with an antibody that blocks only myostatin. They used mdx mice, a model of Duchenne muscular dystrophy. The follistatin did more to ease the disease damage than the antibody. This is direct evidence that blocking activin adds something [21].

Activin A is also linked to cachexia, the muscle wasting seen in cancer. In mice with tumors, blocking the shared ActRIIB receptor reversed the muscle loss, and the mice lived longer. So interest in this pathway reaches past building muscle [17].

Why this version and not FS288

The versions of follistatin differ in length and in a tail at one end. The tail sets how tightly each version grips heparan sulfate, a sugar-coated molecule on cell surfaces.

FS288 grips it tightly. It stays stuck to the cell surface and to the material around the cells where it was made. FS344 grips weakly, so far more of it escapes into the blood [8], [10]. Later work confirmed that this difference shapes what each version does. The versions that travel freely are better placed to act on tissue far from where they were made [10].

Papers give this as the reason FS344 was picked for most muscle gene delivery work. A gene meant to affect muscle across the body needs a protein that can drift away from the cells making it [13].

How it is delivered in studies

Follistatin is a fairly large protein. Injected as a lab-made protein, it does not last long in the blood. For that reason most FS344 studies deliver the gene instead.

The usual method uses AAV1 carrying the FS344 gene with a switch that stays on all the time. It is injected into a muscle or into the bloodstream. After one injection, the animal’s own muscle keeps making follistatin [13], [16].

This is gene therapy. It does not behave like a small drug or a peptide in the body. That is also why anti-doping research on gene doping treats myostatin gene constructs as their own category. The safety and legal sections below say more [14].

What the studies found

The findings below cover FS344 and the wider follistatin, myostatin and activin pathway. They come from mice, monkeys and pigs, plus the one early human trial.

Mice

• 2001. Mice bred to make extra follistatin overcame the limit set by myostatin. They gained a large amount of muscle, more than mice that simply lack working myostatin [3].
• 2007. Extra follistatin was added in mice that already lacked myostatin. They had about four times the muscle of normal mice, some of the largest muscle gains ever reported in mice. It shows the upper limit of blocking both pathways in a lab. It is not a guide to what would happen in a clinic [11].
• 2008. Mdx mice were bred to make a myostatin blocker built from follistatin. They had more muscle, and their muscle tissue showed less disease damage under the microscope [12].
• 2008, a separate study. Mice got a single dose of FS344 by AAV1. Muscle mass and strength stayed higher for the whole time the study tracked them [13].
• 2009. Follistatin grew muscle in adult mice that had already finished growing. It worked by making satellite cells, the muscle’s repair cells, multiply, and by blocking both myostatin and activin. This sets it apart from growth pathways that act only during development [15].
• 2018. In mdx mice, an engineered follistatin that blocks both myostatin and activin grew muscle and eased the disease more than an antibody that blocks myostatin alone [21].

Monkeys and pigs

• 2009. Cynomolgus macaques were given the FS344 gene by AAV1. Muscle mass and strength went up. The study also tracked where the gene went in the body and how well the animals tolerated it. The aim was to prepare for a human study [16].
• 2017. Pigs were bred to make human FS344 in their muscle. Pigs are a larger animal used in farm and medical research. They had more muscle, which takes the evidence past rodents and primates [20].

The one human trial

• The first published human research on FS344 was a phase 1/2a gene therapy trial. A small group of adults with Becker muscular dystrophy had AAV1 carrying FS344 injected straight into muscle. There were six patients. The main measure was the six-minute walk test, a standard measure in nerve and muscle disease. Four of the six walked farther, and two showed no gain. No side effects were reported [18].
• A later review by the same team summed up this trial and the case for follistatin gene therapy [19].
• This work was done only inside an experimental gene therapy trial, in people with one muscle disease. It is not evidence of safety or effect in healthy people, or in any setting other than gene therapy.

Muscle wasting

• 2002. Giving mice myostatin through the whole body was enough, with nothing else, to cause wasting like cachexia. So myostatin signaling can drive muscle loss. This is the pathway follistatin research tries to block [4].
• 2002, a separate study. An antibody against myostatin, not follistatin, improved muscle function in mdx mice. It was early proof that blocking this pathway can help, and later follistatin studies built on it [5].
• 2010. Blocking ActRIIB, the receptor that activin and myostatin share, reversed cancer-linked muscle wasting in mice with tumors and helped them live longer. That supports more research on this pathway in wasting diseases [17].

Safety

What the studies saw

• In the phase 1/2a human trial, the small group generally tolerated the FS344 gene well. The trial’s main purpose was to describe the safety of this experimental gene therapy approach [18].
• Studies in mice, monkeys and pigs did not turn up a repeated safety problem tied to FS344 gene delivery, in the papers reviewed for this guide. Study designs and follow-up times differ a lot from species to species [16], [20].
• Most of this research delivered a gene by AAV1 virus. It did not give the follistatin protein. So the safety findings are about the gene therapy method and the gene package. They say nothing about a lab-made protein or a peptide.

What is not known

• FS344 has not been studied in the general public or in healthy people. The only human data come from a small experimental gene therapy trial in one disease.
• Nobody has published what happens when myostatin and activin stay blocked for longer than the follow-up times in these studies.
• Activin does jobs outside muscle. It plays a part in reproduction, the immune system and other tissues. The wider effects of blocking activin, as opposed to myostatin alone, are not fully worked out in the papers reviewed here.
• No large controlled human trial of FS344 has been published, and none has been repeated by a separate group.
• Gene delivery makes the body produce follistatin on an ongoing basis. There is no set dose that can be stopped. How it moves through the body, and what the safety questions are, differ from those of a normal peptide or protein. Results from one form should not be assumed to hold for the other.

Legal status in the US

• Not FDA approved. No FS344 product has approval for any medical use from the FDA. The same is true of the EMA in Europe and the MHRA in the UK.
• Banned in sport. The World Anti-Doping Agency names follistatin as an example of a myostatin blocker under class S4.3, “Agents preventing Activin receptor IIB activation”. Other myostatin-binding proteins and myostatin antibodies sit in the same class. It is banned at all times, in and out of competition. Nearly all FS344 research uses gene delivery by virus, so it also falls under WADA’s class M3, Gene and Cell Doping. That class bans gene or cell doping to boost performance, as a separate rule [14].
• Research use. It is supplied strictly for laboratory research use.

Limits of the research

1. One small human trial. Only one early human trial has been published. It was a phase 1/2a gene therapy trial with no blinding, in a small group of adults with Becker muscular dystrophy. No larger, controlled or repeated human trial exists.
2. Tied to one delivery method. Almost all the findings are about the FS344 gene delivered by AAV1. They are not about giving lab-made follistatin protein. Results from gene delivery may not hold for other forms.
3. Patients only. All the human data come from people with muscular dystrophy in an experimental trial. No research has been published in healthy people or the general public.
4. Short follow-up. Gene delivery is meant to last. Yet the follow-up in both animal and human studies is fairly short. What happens after those windows is unknown.
5. Few research groups. Much of the FS344 gene delivery work comes from a small number of overlapping groups, the ones who first built the AAV1 and FS344 package. It has not been fully repeated by unrelated labs.
6. Activin does more than muscle. Activin signaling is active in many tissues. The full effect of blocking it with follistatin is still an open research question.

Follistatin 344 built muscle in mice and monkeys when its gene was delivered by virus, and in pigs bred to carry the gene. In people, the evidence is one small trial in one disease.

References

Selected peer-reviewed references, each verified against the CrossRef API before inclusion. Ordered by date of publication.

1. Nakamura T, Takio K, Eto Y, Shibai H, et al. (1990). Activin-Binding Protein from Rat Ovary Is Follistatin. Science, 247(4944), 836–838. DOI: 10.1126/science.2106159
2. McPherron A, Lawler A, Lee S. (1997). Regulation of skeletal muscle mass in mice by a new TGF-β superfamily member. Nature, 387(6628), 83–90. DOI: 10.1038/387083a0
3. Lee S, McPherron A. (2001). Regulation of myostatin activity and muscle growth. Proceedings of the National Academy of Sciences, 98(16), 9306–9311. DOI: 10.1073/pnas.151270098
4. Zimmers T, Davies M, Koniaris L, Haynes P, et al. (2002). Induction of Cachexia in Mice by Systemically Administered Myostatin. Science, 296(5572), 1486–1488. DOI: 10.1126/science.1069525
5. Bogdanovich S, Krag T, Barton E, Morris L, et al. (2002). Functional improvement of dystrophic muscle by myostatin blockade. Nature, 420(6914), 418–421. DOI: 10.1038/nature01154
6. Lee S. (2004). Regulation of Muscle Mass by Myostatin. Annual Review of Cell and Developmental Biology, 20(1), 61–86. DOI: 10.1146/annurev.cellbio.20.012103.135836
7. Amthor H, Nicholas G, McKinnell I, Kemp C, et al. (2004). Follistatin complexes Myostatin and antagonises Myostatin-mediated inhibition of myogenesis. Developmental Biology, 270(1), 19–30. DOI: 10.1016/s0012-1606(04)00118-6
8. Saito S, Sidis Y, Mukherjee A, Xia Y, et al. (2005). Differential Biosynthesis and Intracellular Transport of Follistatin Isoforms and Follistatin-Like-3. Endocrinology, 146(12), 5052–5062. DOI: 10.1210/en.2005-0833
9. Lee S, Reed L, Davies M, Girgenrath S, et al. (2005). Regulation of muscle growth by multiple ligands signaling through activin type II receptors. Proceedings of the National Academy of Sciences, 102(50), 18117–18122. DOI: 10.1073/pnas.0505996102
10. Sidis Y, Mukherjee A, Keutmann H, Delbaere A, et al. (2006). Biological Activity of Follistatin Isoforms and Follistatin-Like-3 Is Dependent on Differential Cell Surface Binding and Specificity for Activin, Myostatin, and Bone Morphogenetic Proteins. Endocrinology, 147(7), 3586–3597. DOI: 10.1210/en.2006-0089
11. Lee S. (2007). Quadrupling Muscle Mass in Mice by Targeting TGF-β Signaling Pathways. PLoS ONE, 2(8), e789. DOI: 10.1371/journal.pone.0000789
12. Nakatani M, Takehara Y, Sugino H, Matsumoto M, et al. (2008). Transgenic expression of a myostatin inhibitor derived from follistatin increases skeletal muscle mass and ameliorates dystrophic pathology in mdx mice. The FASEB Journal, 22(2), 477–487. DOI: 10.1096/fj.07-8673com
13. Haidet A, Rizo L, Handy C, Umapathi P, et al. (2008). Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors. Proceedings of the National Academy of Sciences, 105(11), 4318–4322. DOI: 10.1073/pnas.0709144105
14. Baoutina A, Alexander I, Rasko J, Emslie K. (2008). Developing strategies for detection of gene doping. The Journal of Gene Medicine, 10(1), 3–20. DOI: 10.1002/jgm.1114
15. Gilson H, Schakman O, Kalista S, Lause P, et al. (2009). Follistatin induces muscle hypertrophy through satellite cell proliferation and inhibition of both myostatin and activin. American Journal of Physiology-Endocrinology and Metabolism, 297(1), E157–E164. DOI: 10.1152/ajpendo.00193.2009
16. Kota J, Handy C, Haidet A, Montgomery C, et al. (2009). Follistatin Gene Delivery Enhances Muscle Growth and Strength in Nonhuman Primates. Science Translational Medicine, 1(6), 6ra15. DOI: 10.1126/scitranslmed.3000112
17. Zhou X, Wang J, Lu J, Song Y, et al. (2010). Reversal of Cancer Cachexia and Muscle Wasting by ActRIIB Antagonism Leads to Prolonged Survival. Cell, 142(4), 531–543. DOI: 10.1016/j.cell.2010.07.011
18. Mendell J, Sahenk Z, Malik V, Gomez A, et al. (2015). A Phase 1/2a Follistatin Gene Therapy Trial for Becker Muscular Dystrophy. Molecular Therapy, 23(1), 192–201. DOI: 10.1038/mt.2014.200
19. Al-Zaidy S, Sahenk Z, Rodino-Klapac L, Kaspar B, et al. (2015). Follistatin Gene Therapy Improves Ambulation in Becker Muscular Dystrophy. Journal of Neuromuscular Diseases, 2(3), 185–192. DOI: 10.3233/jnd-150083
20. Chang F, Fang R, Wang M, Zhao X, et al. (2017). The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs. Transgenic Research, 26(1), 25–36. DOI: 10.1007/s11248-016-9985-x
21. Iskenderian A, Liu N, Deng Q, Huang Y, et al. (2018). Myostatin and activin blockade by engineered follistatin results in hypertrophy and improves dystrophic pathology in mdx mouse more than myostatin blockade alone. Skeletal Muscle, 8(1). DOI: 10.1186/s13395-018-0180-z