Follistatin 344 (FS-344) is the name of the 344-amino-acid precursor encoded by the long splice variant of the human follistatin gene. After the cell removes a 29-residue signal peptide, the secreted protein is FS-315, the soluble isoform that circulates in blood, while the shorter FS-288 isoform stays bound to cell surfaces in tissues (Rodino-Klapac 2009). Follistatin binds and neutralizes activins and myostatin (GDF-8), which is why it became a central tool in muscle-growth research. Research-grade Follistatin 344 for laboratory use is supplied for in vitro and animal studies only.
The follistatin gene has an alternative splice site that yields two messenger RNAs. The full-length version encodes a 344-amino-acid preprotein; the other skips exon 6 and encodes a 317-amino-acid preprotein. Both lose a 29-amino-acid signal peptide during secretion, which leaves mature proteins of 315 residues (FS-315, the long isoform) and 288 residues (FS-288, the short isoform) (Rodino-Klapac 2009). A third, intermediate form called FS-303 has been found in ovarian follicular fluid and is thought to arise when the FS-315 tail is clipped (Schneyer 2004).
Follistatin has a basic heparin-binding site at residues 72 to 86. The extra C-terminal tail of FS-315 is rich in acidic amino acids and interferes with that site, so FS-315 binds cell-surface heparan sulfate weakly. FS-288 lacks the tail and binds proteoglycans on cell surfaces (Rodino-Klapac 2009). Measured binding to cell surfaces ranks FS-288 above FS-303 above FS-315 (Schneyer 2004; Sidis 2006).
Schneyer and colleagues tested the prediction that the least sticky isoform would be the one in the circulation. Using an immunoassay specific to full-length FS-315, they found that FS-315 is the major circulating follistatin in human serum and that it was undetectable in follicular fluid samples (Schneyer 2004). That result is the basis for describing FS-315 as circulating and FS-288 as tissue-bound.
Some product descriptions, including an earlier version of this page, call FS-344 “the circulating form” and credit its heparin-binding domain with a longer half-life. The primary literature points the other way on both counts: the circulating protein is FS-315, and the tail on FS-315 reduces heparin binding rather than adding it.
Myostatin, first described as growth/differentiation factor 8 (GDF-8), is a TGF-beta family member expressed in skeletal muscle. Mice with the gene knocked out are much larger than wild-type mice, and individual muscles weigh two to three times more, from a mix of fiber hyperplasia and hypertrophy (McPherron 1997).
Mature myostatin is a disulfide-linked dimer that binds the activin type II receptors, ActRIIB and, to a lesser extent, ActRIIA. In binding experiments, follistatin blocked myostatin from binding ActRIIB (Lee and McPherron 2001). Follistatin was first isolated from the ovary as a suppressor of follicle-stimulating hormone (FSH), and it binds activin, which is how it dampens FSH release (Rodino-Klapac 2009).
Sidis and colleagues produced recombinant FS-288, FS-303 and FS-315 and compared them directly. Activin-binding affinities and kinetics were comparable across the isoforms, and in competition assays myostatin bound the isoforms more potently than BMP-6 or BMP-7, while BMP-2 and BMP-4 did not bind. How well each isoform neutralized myostatin or BMP activity in cells tracked its ability to bind cell-surface proteoglycans (Sidis 2006). In practice, the isoform chosen changes results in cell-based assays.
Because follistatin also binds activins, its effects in vivo are not purely a readout of myostatin blockade. The FSH connection is the main reason gene-therapy groups chose FS-344, which yields the less cell-bound FS-315, in the hope of limiting effects on the pituitary-gonadal axis (Rodino-Klapac 2009).
Se-Jin Lee and Alexandra McPherron at Johns Hopkins built transgenic mice that expressed high levels of one of three myostatin blockers in skeletal muscle: the myostatin propeptide, follistatin, or a dominant-negative ActRIIB. Independent lines for every construct showed dramatic increases in muscle mass comparable to those in myostatin knockout mice (Lee and McPherron 2001). This paper established follistatin as a myostatin antagonist in living animals as well as in binding assays.
If follistatin worked only by blocking myostatin, adding it to mice that already lack myostatin should do nothing. Lee tested this directly. Myostatin-null mice carrying the follistatin transgene had about four times the muscle mass of wild-type mice, with the largest effect in offspring of mothers that were themselves homozygous for the myostatin mutation (Lee 2007). The conclusion was that other TGF-beta ligands with myostatin-like activity also limit muscle growth, and follistatin blocks some of them as well.
That finding means follistatin can change muscle mass beyond what myostatin blockade alone explains (Lee 2007), so its effects on other ligands need attention in any study design.
Before moving to primates, the Nationwide Children’s Hospital group compared three secreted myostatin inhibitors (follistatin-344, FLRG and GASP-1), each delivered once by AAV serotype 1, in wild-type and dystrophic mdx mice. Follistatin-344 produced the largest increases in muscle mass, and in mdx mice treatment improved grip strength, including in animals first treated at about 6.5 months of age. Effects lasted more than two years in wild-type mice, and the authors reported no change in reproductive capacity or cardiac pathology with the FS344 vector (Haidet 2008).
The Nationwide Children’s Hospital group (Kaspar, Mendell and colleagues) injected an AAV serotype 1 vector carrying human FS-344 (AAV1-FS344) into the quadriceps of cynomolgus macaques. Treated muscles showed pronounced, durable increases in size and strength, and long-term transgene expression did not produce abnormal changes in the morphology or function of key organs (Kota 2009). The authors and their institution disclosed a provisional patent on follistatin gene delivery.
In a phase 1/2a proof-of-principle trial, AAV1.CMV.FS344 was injected directly into both quadriceps of six patients with Becker muscular dystrophy, in two dose cohorts of three. The primary outcome in the paper was six-minute walk test (6MWT) distance. In the lower-dose cohort, two patients improved by 58 m and 125 m and one showed no change. In the higher-dose cohort, two improved by 108 m and 29 m and one did not improve. The authors reported no adverse effects, and biopsies showed less endomysial fibrosis, less central nucleation and more normal fiber-size distribution, most clearly at the higher dose (Mendell 2015).
With no placebo group, the trial shows feasibility and a signal rather than proof of benefit.
The same vector, rAAV1.CMV.huFS344, was delivered to both quadriceps of six people with sporadic inclusion body myositis, and all participants also followed an exercise program. Compared with eight untreated patients matched for age, sex and baseline measures, the treated group’s 6MWT distance changed by a median of +56.0 m per year versus -25.8 m per year (p = 0.01). Four treated participants gained 58 to 153 m and two gained 5 to 23 m. Biopsies showed less fibrosis and improved regeneration (Mendell 2017).
Both disease cohorts ran under one registered phase 1 protocol, NCT01519349. Steven Greenberg at Brigham and Women’s Hospital published a critique arguing that the result could not be attributed to follistatin: the comparison group was not a concurrent control, treated patients also received exercise and other interventions, ClinicalTrials.gov listed safety rather than the 6MWT as the primary outcome, the “annualized” walking metric depended on which time point was chosen, and adverse events were expressed as a share of all events rather than of patients (Greenberg 2017). A fair reading is that the inclusion body myositis data are hypothesis-generating and have not been confirmed in a randomized trial.
ACE-083 is Acceleron’s locally acting, follistatin-based therapeutic that binds myostatin and other muscle regulators. It is an engineered fusion protein, not native follistatin. In a phase 1, randomized, double-blind, placebo-controlled study in 58 healthy postmenopausal women (42 on ACE-083, 16 on placebo), injection into the rectus femoris or tibialis anterior produced maximum mean muscle-volume increases of 14.5% and 8.9% at the highest dose, with no serious adverse events and no significant change in mean muscle strength (Glasser 2018).
Two phase 2 trials followed. In facioscapulohumeral muscular dystrophy, total muscle volume rose 16.4% versus placebo (P = .01), but there were no consistent improvements in functional or patient-reported outcomes over up to 12 months (Statland 2022). In Charcot-Marie-Tooth disease, total muscle volume in the injected tibialis anterior rose 13.5% versus placebo (p = 0.0096), but fat fraction and functional outcomes did not consistently improve (Thomas 2022). In both studies the most common adverse events were mild or moderate injection-site reactions.
Follistatin is one of several ways to probe the myostatin/activin axis. The table below compares it with the recombinant ligand itself and with two agents that reached human testing.
| Tool | What it is | Targets | Key published finding | Typical research use |
|---|---|---|---|---|
| Follistatin (FS-344 precursor, FS-315 / FS-288 proteins) | Secreted glycoprotein, 288 to 315 residues when mature | Activins, myostatin, some BMPs | Muscle transgene increased mass to levels comparable with myostatin knockouts (Lee and McPherron 2001) | Ligand-trap experiments, isoform comparisons, AAV gene-transfer models |
| ACE-031 | Soluble ActRIIB fused to IgG1-Fc | Myostatin and related ActRIIB ligands | DMD trial stopped early over epistaxis and telangiectasias; lean-mass and 6MWT trends were not statistically significant (Campbell 2017) | Receptor-decoy comparisons with broader ligand coverage |
| Myostatin antibodies (e.g., MYO-029) | Neutralizing monoclonal antibody | Myostatin | 116-subject phase 1/2 trial: no strength or function improvement (not powered for efficacy); skin hypersensitivity at higher doses (Wagner 2008) | Selective myostatin blockade as a comparator to broader traps |
| GDF-8 (myostatin) | The ligand itself, a disulfide-linked C-terminal dimer | Signals via ActRIIB and, less strongly, ActRIIA | Knockout mice have muscles two to three times heavier (McPherron 1997) | Positive control and binding partner in inhibitor assays |
A common assay layout pairs recombinant GDF-8 myostatin or myostatin 1 mg as the ligand with follistatin or a receptor decoy such as ACE-031 as the inhibitor, then reads out SMAD2/3 signaling or myoblast differentiation. Because follistatin also binds activins, including an activin control helps separate myostatin-specific effects from broader ligand trapping.
MYO-029 had good safety and tolerability apart from cutaneous hypersensitivity at the two highest doses, and a limited number of participants showed a trend toward increased muscle size on DEXA and histology (Wagner 2008). ACE-031 in boys with Duchenne muscular dystrophy caused no serious adverse events, but the study was halted after the second dosing regimen because of nosebleeds and telangiectasias, and the authors noted that these non-muscle adverse events contributed to the decision to discontinue (Campbell 2017). Together these results show the trade-off in this pathway: selective myostatin blockade was well tolerated but showed no functional benefit, while the broader ActRIIB decoy was stopped over vascular adverse events.
They are linked but not identical. FS-344 is the 344-residue precursor. Removing the 29-residue signal peptide gives the secreted 315-residue protein, FS-315 (Rodino-Klapac 2009).
FS-288 lacks the acidic C-terminal tail, binds cell-surface proteoglycans strongly and is found in tissue compartments. FS-315 binds cell surfaces weakly and is the major form in human serum (Schneyer 2004).
We found no published human trial of native follistatin protein. Human data come from AAV1.CMV.FS344 gene transfer and from ACE-083, an engineered follistatin-based fusion protein. The gene-transfer cohorts in Becker muscular dystrophy and inclusion body myositis reported walking-distance gains in some participants, but neither was randomized, and the inclusion body myositis results were publicly disputed (Greenberg 2017).
No. Follistatin still increased muscle mass in mice that lacked myostatin entirely, which points to other TGF-beta ligands it also blocks (Lee 2007).
Peptides For Sale supplies Follistatin 344 (1 mg) as a lyophilized research material with a Certificate of Analysis per batch. Contact us for the expressed sequence and isoform of the current lot. Match the isoform and assay format to the questions above before ordering, and pair it with the appropriate ligand controls.
Research use only. All products sold on this site are intended strictly for in vitro laboratory research and preclinical study by qualified professionals. They are not drugs, foods, dietary supplements or cosmetics, are not intended for human or veterinary use, and must not be used to diagnose, treat, cure or prevent any disease. The clinical studies described here tested investigational agents (AAV gene-therapy vectors, ACE-083, MYO-029 and ACE-031), not research-grade follistatin, and nothing in this article is medical advice or guidance for personal use.
