FLGR-242: What Is Known About the Claimed Follistatin Analogue

Key facts

• Claimed type: recombinant (lab-made) follistatin analogue; a protein, not a short peptide.
• Claimed target: myostatin (GDF-8) and activins, which signal through activin type II receptors.
• Published studies on FLGR-242: none on PubMed and no trials on ClinicalTrials.gov (checked 15 October 2026).
• Published sequence or structure: none.
• Approval status (October 2026): not approved as a medicine anywhere; no developer has been publicly identified.
• Sport: follistatin and other myostatin-binding proteins are prohibited at all times under section S4.3 of the 2026 WADA Prohibited List.

Every so often a compound becomes widely talked about before anyone has published anything about it. FLGR-242 is a clear example. This article sets out what the science of follistatin and myostatin actually shows, what is missing for FLGR-242, and why the gap matters for anyone trying to judge it as a research material.

Why is FLGR-242 being talked about?

During 2026 the name FLGR-242, sometimes written FLG-242, began to appear in US fitness forums, podcasts and social media posts, and on websites offering it for research use. The interest is driven by claims, not data. Seller descriptions do not agree with each other on basic points such as how long it lasts in the body, how it is built or how strongly it affects activins. None of those claims is linked to a published study.

The appeal is easy to see. Myostatin is the best-known natural brake on muscle growth, and follistatin is the body's own protein for neutralising it. A product that promises to release that brake attracts attention in bodybuilding circles. One seller page we reviewed concedes that the primate and human data it cites come from follistatin gene therapy studies, not from the product it sells.

Even the name is unclear. FLG is the official gene symbol for filaggrin, an unrelated skin protein, and FLRG is an older name for follistatin-like 3 (FSTL3), a real follistatin relative that also binds myostatin. Neither tells us anything about what FLGR-242 contains.

What is follistatin?

Follistatin was first isolated in 1987 from ovarian fluid, as a protein that suppressed the release of follicle-stimulating hormone, which is where its name comes from (Ueno et al., 1987). The human gene produces two precursor forms by alternative splicing, of 344 and 317 amino acids, known as FS344 and FS317 (Shimasaki et al., 1988). After processing these give the circulating and tissue forms usually called FS315 and FS288, which differ in how strongly they stick to cell surfaces.

Follistatin works by binding and neutralising several members of the TGF-beta family, including activins, myostatin and related growth factors (Lee et al., 2010). It is not a muscle-specific protein. Mice bred without follistatin die within hours of birth, with defects in muscle, skeleton, skin and whiskers (Matzuk et al., 1995). Activins and follistatin are also involved in reproduction, inflammation and tissue scarring (Hedger and de Kretser, 2013).

What does the myostatin research show?

Myostatin was described in 1997, when mice lacking the gene were found to have roughly two to three times the usual skeletal muscle mass (McPherron et al., 1997). Natural loss-of-function mutations explain the heavily muscled Belgian Blue cattle and were reported in a child with marked muscle hypertrophy (Schuelke et al., 2004).

In mice, making muscle produce extra follistatin caused dramatic increases in muscle mass (Lee and McPherron, 2001). When the same follistatin transgene was added to mice that already lacked myostatin, they had about four times the muscle mass of normal mice, which showed that other signals with a similar braking effect exist (Lee, 2007). Later work pointed to activins as part of that group (Lee et al., 2010). That broader reach is exactly why follistatin is studied, and also why its effects outside muscle need care. Our article on bimagrumab and other myostatin blockers covers the medicines that target this pathway in more detail.

What did the follistatin gene therapy studies find?

Most of the human-relevant follistatin data come from gene therapy, in which a harmless virus (AAV) carries the FS344 gene into muscle so that the muscle makes follistatin itself, locally and for a long time.

• Monkeys: delivering the FS344 gene into the thigh muscles of cynomolgus macaques increased the size and strength of the treated muscles (Kota et al., 2009).
• Becker muscular dystrophy: six patients received the gene into both thigh muscles in a phase 1/2a trial with no placebo group. Four of the six walked further in a six-minute walk test, two did not change, and no adverse effects were reported (Mendell et al., 2015).
• Inclusion body myositis: six patients received the same approach, alongside an exercise programme, and were compared with eight untreated patients. The treated group improved on the six-minute walk test while the untreated group declined (Mendell et al., 2017).

These are small, early studies in people with serious muscle disease, and none has led to an approved treatment. They also tell us nothing direct about a follistatin protein supplied in a vial, for the reasons below.

Why is a follistatin protein in a vial a different problem?

Natural follistatin does not last long in the blood. When drug developers tried to make a usable follistatin medicine, they found the native protein was cleared too quickly and had to re-engineer it, for example by fusing it to an antibody fragment and changing the region that binds to cell surfaces, which extended its half-life in animals about 100-fold (Datta-Mannan et al., 2013). Each change of this kind alters where the protein goes, how long it stays and what it binds, so every engineered version needs its own studies. None has been published for FLGR-242.

Medicines that block the same pathway also show why effects outside muscle matter:

• ACE-031, a decoy receptor that mops up myostatin and related signals, was tested in boys with Duchenne muscular dystrophy. The trial was stopped after the second dose level because of nosebleeds and small dilated blood vessels (telangiectasias), even though no serious adverse events were recorded (Campbell et al., 2017).
• Sotatercept, an activin-trapping protein now used in pulmonary arterial hypertension, also showed nosebleeds, telangiectasia and raised haemoglobin in its phase 3 trial (Hoeper et al., 2023). This points to activin signalling in blood vessels and blood cell production.
• Bimagrumab, an antibody that blocks activin type II receptors, did not improve walking distance in a phase 2b trial of 251 people with inclusion body myositis (Hanna et al., 2019). More muscle does not automatically mean better function.
• Apitegromab, which targets only the inactive precursor of myostatin, met its main goal in a phase 3 trial in spinal muscular atrophy (Crawford et al., 2025) and was approved by the US FDA in September 2026 for that disease. Its developers chose a narrow target partly to avoid hitting activins.

What is not known about FLGR-242?

• What it is. No amino acid sequence, structure, molecular weight or manufacturer has been published, so no one can confirm that a given vial contains what the label says.
• What it binds. Claims that it spares activins, or only partly blocks them, have not been published or tested in public. This matters because blocking activins affects tissues well beyond muscle.
• How long it lasts and where it goes. There are no published pharmacokinetic data, and native follistatin shows that this cannot be assumed.
• Safety. There are no published animal toxicology or human safety data. The related medicines above show that blocking this pathway can affect blood vessels, blood cells and reproduction.
• Stability. Large proteins can unfold, clump together (aggregate) or lose activity in storage. No stability data have been published.

How would a lab confirm what a follistatin-type protein is?

A recombinant follistatin is a protein of roughly 30 to 40 kDa, more than ten times the size of a typical short research peptide, and natural follistatin carries sugar chains (glycosylation). That makes identity and purity much harder to confirm than for a short peptide. A single HPLC purity figure is not enough. A lab would normally need:

• intact mass by mass spectrometry, compared against a published or declared sequence;
• peptide mapping (digesting the protein and matching the fragments) to confirm the sequence;
• size-exclusion chromatography or gel electrophoresis to look for aggregates and fragments;
• a check of glycosylation, if the protein is meant to carry it;
• a binding or activity assay against myostatin and activin A, to show the protein actually works as claimed.

Without a published sequence to compare against, the first two steps have nothing to check against. Our guide on how to read and verify a certificate of analysis explains what good testing looks like for research materials in general.

Is FLGR-242 banned in sport?

Yes, as a class. Section S4.3 of the 2026 WADA Prohibited List bans agents that prevent activin receptor IIB activation at all times, and names myostatin-binding proteins such as follistatin and myostatin propeptide as examples, alongside decoy receptors and antibodies against myostatin or its receptors (WADA, 2026). Gene transfer of follistatin is banned separately under gene doping.

Bottom line

The biology behind the interest in FLGR-242 is real: myostatin limits muscle growth, and follistatin can neutralise it. But the evidence people quote comes from mice, from follistatin gene therapy in monkeys and in a dozen patients, and from medicines that are different molecules. For FLGR-242 itself there is no published identity, no pharmacology and no safety data. Until there is, it cannot be assessed as a research material, and claims made for it should be read as marketing.

References

1. Campbell C, et al. (2017). Myostatin inhibitor ACE-031 treatment of ambulatory boys with Duchenne muscular dystrophy: results of a randomized, placebo-controlled clinical trial. Muscle and Nerve. PubMed 27462804
2. Crawford TO, et al. (2025). Safety and efficacy of apitegromab in nonambulatory type 2 or type 3 spinal muscular atrophy (SAPPHIRE): a phase 3, double-blind, randomised, placebo-controlled trial. The Lancet Neurology. PubMed 40818473
3. Datta-Mannan A, et al. (2013). An engineered human follistatin variant: insights into the pharmacokinetic and pharmocodynamic relationships of a novel molecule with broad therapeutic potential. Journal of Pharmacology and Experimental Therapeutics. PubMed 23249626
4. Hanna MG, et al. (2019). Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial. The Lancet Neurology. PubMed 31397289
5. Hedger MP, de Kretser DM (2013). The activins and their binding protein, follistatin: diagnostic and therapeutic targets in inflammatory disease and fibrosis. Cytokine and Growth Factor Reviews. PubMed 23541927
6. Hoeper MM, et al. (2023). Phase 3 trial of sotatercept for treatment of pulmonary arterial hypertension. New England Journal of Medicine. PubMed 36877098
7. Kota J, et al. (2009). Follistatin gene delivery enhances muscle growth and strength in nonhuman primates. Science Translational Medicine (a correction was published in 2026). PubMed 20368179
8. Lee SJ (2007). Quadrupling muscle mass in mice by targeting TGF-beta signaling pathways. PLoS One. PubMed 17726519
9. Lee SJ, McPherron AC (2001). Regulation of myostatin activity and muscle growth. Proceedings of the National Academy of Sciences. PubMed 11459935
10. Lee SJ, et al. (2010). Regulation of muscle mass by follistatin and activins. Molecular Endocrinology. PubMed 20810712
11. Matzuk MM, et al. (1995). Multiple defects and perinatal death in mice deficient in follistatin. Nature. PubMed 7885475
12. McPherron AC, et al. (1997). Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member. Nature. PubMed 9139826
13. Mendell JR, et al. (2015). A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy. Molecular Therapy. PubMed 25322757
14. Mendell JR, et al. (2017). Follistatin gene therapy for sporadic inclusion body myositis improves functional outcomes. Molecular Therapy. PubMed 28279643
15. Schuelke M, et al. (2004). Myostatin mutation associated with gross muscle hypertrophy in a child. New England Journal of Medicine. PubMed 15215484
16. Shimasaki S, et al. (1988). Primary structure of the human follistatin precursor and its genomic organization. Proceedings of the National Academy of Sciences. PubMed 3380788
17. Ueno N, et al. (1987). Isolation and partial characterization of follistatin: a single-chain Mr 35,000 monomeric protein that inhibits the release of follicle-stimulating hormone. Proceedings of the National Academy of Sciences. PubMed 3120188
18. World Anti-Doping Agency (2026). The 2026 Prohibited List, section S4.3. wada-ama.org