What Is Follistatin 344? The Activin- and Myostatin-Binding Protein, Explained

Published October 2026 · Patriot Labs Research Team · ~16 min read

The short answer

Follistatin 344 is the full-length, 344-amino-acid precursor form of follistatin, a secreted glycoprotein that binds and neutralizes activins and myostatin, two members of the TGF-β growth-factor family. Removing its 29-residue signal sequence leaves the 315-residue mature protein, FS315.^[2][3]

Follistatin was first isolated in 1987 as a protein that suppressed follicle-stimulating hormone (FSH), which is where its name comes from.^[1] It is now best known as a ligand trap. Two follistatin molecules wrap around an activin dimer and block the surfaces activin uses to reach its receptors.^[5] Because it also binds myostatin, the growth factor that restrains skeletal-muscle growth, it became a central tool in muscle-biology research, and the gene encoding FS344 has been used in animal and early clinical gene-therapy studies.^{[7][10][11][12]}

The rest of this guide explains each of those ideas in turn and where the evidence stops.

What “344” means: FS344, FS315 and FS288

The number refers to amino-acid count, and it names the precursor rather than the protein that ends up circulating. The human follistatin gene, FST, encodes a 344-amino-acid protein whose first 29 residues are a signal sequence, a short leader that directs the protein out of the cell and is then cut off.^[2][3]

So “Follistatin 344” is a reference to the full-length sequence. In the macaque gene-therapy study discussed below, the authors note explicitly that the FS344 construct encodes the FS315 isoform: the cell makes the 344-residue precursor, removes the signal sequence, and secretes FS315.^[11]

The difference between FS315 and FS288 matters biologically. The C-terminal acidic tail present in FS315 appears to dampen binding to heparan sulfate on cell surfaces. In a 1993 study of ovarian granulosa cells, Sugino and colleagues reported that the C-terminally truncated forms of follistatin bound heparan sulfate proteoglycans with higher affinity.^[4] In practical terms, the shorter form tends to stay near the cells that make it, while the full-length form circulates.

How follistatin was discovered

Follistatin was found while researchers were looking for regulators of FSH. In 1987, Ueno, Ling, Guillemin and colleagues isolated it from porcine ovarian follicular fluid. They described it as a single-chain monomeric protein of about 35,000 Da (with a less abundant 32,000 Da form) that inhibited basal FSH secretion, but not luteinizing hormone (LH) secretion, in cultured rat pituitary cells.^[1] The name combines “follicle” and “statin,” from its FSH-suppressing activity.

A year later, Shimasaki and colleagues reported the primary structure of the human follistatin precursor and the organization of its gene.^[2] This is the work that established the 344-residue precursor and showed how alternative splicing produces the shorter form.

The explanation for the FSH effect came next. Follistatin does not act on the pituitary directly as a hormone. It works by binding activin, a growth factor that stimulates FSH, and taking it out of circulation. By 1994 follistatin was routinely described as an activin antagonist, and it had turned up somewhere unexpected: Hemmati-Brivanlou, Kelly and Melton found it in the Spemann organizer of frog embryos, where it converted ectodermal tissue to neural tissue by blocking activin signaling.^[8] That finding made follistatin a standard tool in developmental biology as well as reproductive endocrinology.

How the protein is built

Follistatin is a cysteine-rich glycoprotein with a modular structure: an N-terminal domain followed by three repeated “follistatin domains” (FS1, FS2 and FS3), and in FS315 a C-terminal acidic tail. The first follistatin domain carries a heparin-binding site.^[3] The protein is held together by many internal disulfide bonds and carries N-linked sugars, which is why it runs larger than its sequence predicts. A commercial reference datasheet gives a predicted mass of 34.7 kDa but an observed range of 39–56 kDa on a reducing SDS-PAGE gel, the spread coming from glycosylation.^[3]

How follistatin traps activin and myostatin

Follistatin is a binding protein, not a receptor agonist. It does not switch a pathway on. It neutralizes a growth factor by wrapping around it so the growth factor cannot reach its receptors.

Activin, like other TGF-β family members, signals by assembling two kinds of receptor on the cell surface: a type II receptor, which binds first, and a type I receptor, which it then recruits. In 2005, Thompson, Lerch, Cook, Woodruff and Jardetzky solved the crystal structure of follistatin bound to activin A at 2.8 Å resolution. The structure shows two follistatin molecules encircling one activin dimer. Together they cover the binding sites for both receptor types, which is the structural explanation for how follistatin shuts activin signaling down so completely.^[5]

This “ligand trap” model is why follistatin is useful in research. Adding follistatin to a cell or tissue system removes free activin, or myostatin, from the picture, letting investigators ask what those growth factors were doing in the first place.

The myostatin connection

Most current interest in follistatin comes from its link to myostatin, the growth factor that acts as a brake on skeletal-muscle growth.

Myostatin, also called GDF-8, was identified in 1997 by McPherron, Lawler and Lee. They found a new TGF-β family member expressed mainly in skeletal muscle, and mice engineered to lack it had substantially heavier muscles, apparently from both more and larger muscle fibers.^[6] That established myostatin as a negative regulator of muscle mass.

In 2001, Lee and McPherron tied follistatin directly into that pathway. In their experiments, follistatin blocked myostatin from binding its receptor, the activin type IIB receptor (ActRIIB). Transgenic mice engineered to overexpress follistatin in skeletal muscle showed large increases in muscle mass, comparable to those seen in myostatin-knockout mice.^[7] Because follistatin binds activins as well as myostatin, the result also suggested that more than one TGF-β family ligand contributes to limiting muscle growth.

These are genetic findings in mice. They show what happens when the follistatin–myostatin balance is changed by engineering the animal’s own genes, and they are the reason follistatin became a reference molecule in muscle-biology research. They are not studies of a protein administered from a vial.

What knockout and developmental studies showed

Removing follistatin entirely shows how broadly it acts. In 1995, Matzuk and colleagues reported that mice lacking the follistatin gene were growth-retarded and had reduced diaphragm and intercostal muscle mass. They also had abnormalities of the skin, hard palate, ribs, whiskers and teeth, and they died within hours of birth.^[9] Because those defects were more widespread than those seen in activin-deficient mice, the authors concluded that follistatin modulates several TGF-β family members, not activin alone.^[9]

Together with the frog-embryo work described above, this places follistatin as a general regulator of growth-factor availability across development, not a single-purpose hormone.^[8][9]

FS344 in gene-therapy research

The name “FS344” is most common in the gene-therapy literature, where researchers delivered the follistatin gene to muscle with adeno-associated virus (AAV) vectors so that the muscle itself would produce and secrete follistatin.

• Mice (2008). Haidet and colleagues reported long-term increases in skeletal muscle mass and strength in mice after a single gene administration of myostatin inhibitors, including follistatin.^[10]
• Non-human primates (2009). Kota and colleagues injected an AAV1 vector carrying human FS344 into the quadriceps of cynomolgus macaques, with the opposite leg as an internal control. They reported increases in quadriceps size and fiber hypertrophy that were largely maintained over the study. They found no abnormal changes in liver, kidney, heart or blood measures, and reproductive hormones stayed close to baseline. The study was small, three animals per group, and used healthy animals rather than a disease model.^[11]
• Early clinical trial. Mendell and colleagues reported a phase 1/2a proof-of-principle trial in six patients with Becker muscular dystrophy. It used an AAV1.CMV.FS344 vector injected directly into the quadriceps, with the 6-minute walk test as the primary outcome. The authors reported no adverse effects and described the results as encouraging, but it was a small, uncontrolled study.^[12] A related trial in Duchenne muscular dystrophy was registered on ClinicalTrials.gov.^[13]

Two points matter when reading this literature. First, every one of these studies delivered a gene, not follistatin protein: the muscle made its own follistatin continuously, locally, for months. That is a fundamentally different situation from a protein sitting in a vial. Second, none of it has produced an approved medicine. Follistatin is not an FDA-approved drug in any form, and the research-grade protein is not the material used in these studies.

A protein, not a peptide: why that matters

Most research peptides are short chains of a few to a few dozen amino acids made by solid-phase synthesis. Follistatin is a 315-residue, glycosylated, disulfide-rich protein. Proteins of this size and complexity are normally produced by expression in living cells rather than by chemical synthesis. Our guide to recombinant vs synthetic peptides explains the difference.

That has practical consequences:

• Folding matters. Follistatin’s activity depends on its three-dimensional fold and correctly paired disulfide bonds, not just its sequence. A correct mass and sequence do not, on their own, prove a correctly folded protein.
• Glycosylation varies. The sugar groups depend on the cells that produced the protein, so different preparations can differ in apparent size and in how they behave on a gel or in solution.^[3]
• Analysis is harder. Purity and identity testing for a glycoprotein of this size is more involved than for a short peptide. See Understanding Peptide COA Testing and What Is Mass Spectrometry for how certificates are read.

In the laboratory

Research follistatin is supplied as a lyophilized powder. Proteins are generally less forgiving than short peptides, so a few handling points carry extra weight:

• Keep it cold and dry. Store the sealed vial refrigerated, protected from light and moisture. See cold-chain storage.
• Avoid freeze–thaw cycles. Large folded proteins are particularly prone to aggregation and loss of structure with repeated freezing and thawing. See how peptides degrade.
• Mix gently. Vigorous shaking can denature proteins at air–liquid surfaces. The general principles are in How to Reconstitute Research Peptides and Peptide Handling Best Practices.

For reference, Patriot Labs is a first-responder-owned, US-based company. Its batches are tested by independent US laboratories, and certificates of analysis are published on product pages as they are issued.

What this guide does not establish

This guide is an educational account of follistatin’s biology and research history. Several limits are worth stating plainly.

• Animal findings stay in their models. The muscle results come from genetically engineered mice and from AAV gene delivery in mice and a small number of macaques. They describe what happened in those systems, not what any material does elsewhere.
• Gene therapy is not protein. The FS344 studies delivered a gene that made muscle produce follistatin continuously and locally. They say nothing directly about a protein supplied as a powder.
• The clinical work is early and small. The Becker muscular dystrophy trial enrolled six people with no placebo group. It is research history, not evidence of an approved treatment, and follistatin is not an FDA-approved drug.
• Follistatin acts broadly. It binds several TGF-β family members, and knockout mice show defects across many tissues.^[9] Its effects cannot be reduced to “myostatin inhibition” alone.
• Nothing here concerns use in a person or animal. This guide gives no amounts, schedules or methods of use for any product.

Frequently asked questions

What is Follistatin 344?

Follistatin 344 is the full-length, 344-amino-acid precursor form of follistatin, a glycoprotein that binds and neutralizes activins and myostatin. After its 29-residue signal sequence is removed, the mature protein is the 315-residue FS315 isoform.

What is the difference between FS344, FS315 and FS288?

FS344 is the full precursor including the signal sequence. FS315 is the mature full-length protein that circulates in plasma. FS288 is a shorter splice variant that lacks the C-terminal acidic tail, binds activin most tightly, and binds cell-surface heparan sulfate more strongly.

Does follistatin inhibit myostatin?

In laboratory studies, yes. Follistatin binds myostatin and blocks it from reaching its activin type IIB receptor, and transgenic mice that overexpress follistatin in muscle showed large increases in muscle mass, similar to myostatin-knockout mice. These are findings in engineered mice, not in any administered product.

How does follistatin block activin?

Two follistatin molecules wrap around one activin dimer and cover the binding sites for both the type I and type II receptors, so activin cannot signal. This was shown in a 2005 crystal structure of the follistatin–activin complex.

Is follistatin a peptide or a protein?

It is a protein: a 315-residue, glycosylated, disulfide-rich glycoprotein with an apparent mass of roughly 35 kDa or more. That makes it far larger than a typical research peptide, and it is normally produced by expression in cells rather than by chemical peptide synthesis.

Why is it called follistatin?

It was discovered in 1987 in ovarian follicular fluid as a protein that suppressed the release of follicle-stimulating hormone (FSH). It does so indirectly, by binding activin, which stimulates FSH.

Is Follistatin 344 FDA approved?

No. Follistatin is not an FDA-approved drug in any form. FS344 gene-therapy constructs have been tested in small early-stage clinical trials, but research-grade follistatin is sold strictly for in-vitro research and laboratory use and is not for human or veterinary use.

References

1. Ueno, N., Ling, N., Ying, S.-Y., Esch, F., Shimasaki, S., & Guillemin, R. (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, 84(23), 8282–8286. doi:10.1073/pnas.84.23.8282
2. Shimasaki, S., Koga, M., Esch, F., et al. (1988). Primary structure of the human follistatin precursor and its genomic organization. Proceedings of the National Academy of Sciences, 85(12), 4218–4222. doi:10.1073/pnas.85.12.4218
3. R&D Systems. Recombinant Human Follistatin (catalog 4889-FN) product datasheet: precursor, signal sequence, isoforms, heparin-binding domain and apparent molecular mass. https://resources.rndsystems.com/pdfs/datasheets/4889-fn.pdf
4. Sugino, K., Kurosawa, N., Nakamura, T., et al. (1993). Molecular heterogeneity of follistatin, an activin-binding protein. Higher affinity of the carboxyl-terminal truncated forms for heparan sulfate proteoglycans on the ovarian granulosa cell. Journal of Biological Chemistry. PMID: 8340384
5. Thompson, T. B., Lerch, T. F., Cook, R. W., Woodruff, T. K., & Jardetzky, T. S. (2005). The structure of the follistatin:activin complex reveals antagonism of both type I and type II receptor binding. Developmental Cell, 9, 535–543. doi:10.1016/j.devcel.2005.09.008 (PDB 2B0U)
6. McPherron, A. C., Lawler, A. M., & Lee, S.-J. (1997). Regulation of skeletal muscle mass in mice by a new TGF-β superfamily member. Nature, 387(6628), 83–90. doi:10.1038/387083a0
7. Lee, S.-J., & McPherron, A. C. (2001). Regulation of myostatin activity and muscle growth. Proceedings of the National Academy of Sciences, 98, 9306–9311. PMID: 11459935
8. Hemmati-Brivanlou, A., Kelly, O. G., & Melton, D. A. (1994). Follistatin, an antagonist of activin, is expressed in the Spemann organizer and displays direct neuralizing activity. Cell, 77(2), 283–295. PMID: 8168135
9. Matzuk, M. M., Lu, N., Vogel, H., et al. (1995). Multiple defects and perinatal death in mice deficient in follistatin. Nature, 374, 360–363. doi:10.1038/374360a0
10. Haidet, A. M., Rizo, L., Handy, C., 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, 4318–4322.
11. Kota, J., Handy, C. R., Haidet, A. M., 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
12. Mendell, J. R., Sahenk, Z., Malik, V., et al. (2014, published online). A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy. Molecular Therapy. PMID: 25322757
13. ClinicalTrials.gov. Clinical Intramuscular Gene Transfer of rAAV1.CMV.huFollistatin344 Trial to Patients With Duchenne Muscular Dystrophy. NCT02354781. https://clinicaltrials.gov/study/NCT02354781

For related reading, see Recombinant vs Synthetic Peptides, What Is Glycosylation?, What Is a Disulfide Bond? and HPG-Axis Peptides, which covers the FSH and reproductive-hormone side of the story.

All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human or veterinary consumption. This guide is educational and describes the biology, structure and published research history of follistatin in general terms; it is not medical advice, does not describe how to use any product, and makes no claim about outcomes in humans or animals. Gene-therapy studies are described as research history only; research-grade follistatin is not an approved drug and is not the material used in those studies.