Follistatin: The Complete Guide

Follistatin (FST) is an endogenous secreted glycoprotein that binds and neutralizes activins and other TGF-β-superfamily ligands including myostatin (GDF-8) and certain BMPs. First cloned from porcine ovarian fluid in 1987 (Ueno, Ling et al., PMID 3153465) — originally identified as a follicle-stimulating-hormone-release-inhibiting polypeptide, hence the name.

Follistatin is not a peptide in any small-molecule sense. The mature secreted protein is roughly 35–37 kDa (~315–344 amino acids), with multiple cysteine-rich domains and N-glycosylation.

Three isoforms (audit-critical disambiguation):

Isoform	aa	Origin	Function
FS344	344	Long mRNA splice variant; pre-pro-protein	Translated to FS317 after signal-peptide cleavage; further proteolyzed in vivo to FS315. Lower activin affinity than FS288.
FS315	315	Long mRNA + C-terminal proteolytic cleavage	Predominant serum / circulating form. Lower affinity. Reduced heparin binding.
FS288	288	Short mRNA splice variant (no acidic tail)	Cell-surface bound (binds heparan-sulfate proteoglycans). Highest activin affinity.

Note: some encyclopedia content lists isoforms as “FS288, FS303, FS315.” That’s wrong. The standard modern triumvirate is FS288, FS315, FS344. FS303 is not the standard third name.

Regulatory status:

• FDA (US): not approved for any indication. No NDA, no BLA. Not on the FDA 503A bulks list. Sold as research chemical only. Multiple INDs have existed (Acceleron’s ACE-083, Sarepta/Nationwide’s AAV1-FS344 / SRP-9003) — none have produced an approved product.
• EMA (EU): not approved.
• WADA 2026: prohibited at all times under S2.1.4 myostatin function modifiers. Note this is a specific WADA classification with dedicated detection methodology — not just an S0 catch-all.
• IFHA (horseracing): Article 6 prohibition in racehorses (PMID 37401514).
• NCAA, NFL, MLB — banned class.

Follistatin binds and sequesters TGF-β-superfamily ligands, preventing them from engaging their type I and type II receptors.

Primary binding partners (verified):

• Activin A and activin B — primary historical target. Crystal structure of follistatin:activin complex: Thompson et al. Dev Cell 2005 (PMID 16198295). Two follistatin molecules wrap a single activin dimer and block both type I and type II receptor binding.
• Myostatin (GDF-8) — confirmed binding via crystal structure. Cash et al. EMBO J 2009 (PMID 19644449) solved the myostatin:FS288 complex; similar two-follistatin "necklace" topology around myostatin.
• BMP-2, BMP-4, BMP-7 — weaker binding; some BMPs are sequestered.

Downstream of myostatin neutralization:

• Myostatin normally signals via ActRIIB → ALK4/5 → SMAD2/3 phosphorylation, suppressing muscle protein synthesis.
• Follistatin sequestration of myostatin reduces SMAD2/3 phosphorylation → derepresses muscle protein synthesis.
• Net effect: increased muscle mass / lean tissue, reduced fibrosis (in some models).

The endogenous-regulation framing:

Park et al. 2026 (PMID 41863133) showed combined resistance exercise + essential amino acid intake enhances the endogenous follistatin/myostatin ratio in older women — supporting the framing that follistatin is part of a normal regulatory loop, not a pharmacological intervention.

This is where vendor copy and the published clinical record diverge most consequentially.

The Mendell / Nationwide Children’s / Sarepta program (real clinical evidence):

PMID	Year	Population	Result
25322757	2015	n=6 BMD patients (Becker MD)	First-in-human IM injection of AAV1-FS344 (gene therapy) into quadriceps. 4/6 showed improved 6-minute walk test; reduced fibrosis on biopsy; no immune-mediated AEs.
27858738	2015	BMD trial follow-up	Reviews FS344-vs-FS288 isoform rationale; ambulation improvements sustained.
28279643	2017	n=6 sIBM patients	IM AAV1-FS344 in quadriceps; mixed but generally encouraging functional outcomes.

The clinical program now markets as SRP-9003 under Sarepta. Phase 3 is in progress (advanced into LGMD-2E rather than primarily BMD). The route is direct intramuscular AAV1 vector injection — the patient’s own muscle then manufactures FS344 protein locally and secretes it as the FS315 serum form.

Why FS344 specifically? The Mendell group has explicitly chosen FS344 for gene therapy because its post-translational conversion to FS315 yields the lower-activin-affinity serum form — avoiding the activin-mediated suppression of FSH that would result from FS288-style high-affinity binding. That nuance — FS344 is preferred for clinical use because it produces FS315, not because the FS344 form itself is the active species — is almost universally lost in vendor copy.

The vendor "FST-344" reality (per WADA forensic analysis):

Black-market "FST-344" vials are purportedly His-tagged recombinant FS344 protein dosed by injection — a route never used in any human clinical trial.

Reichel et al. 2019 (PMID 31758732), a WADA-accredited forensic analysis: tested 17 black-market "follistatin 344" products.

• Only 9 of 17 actually contained follistatin.
• Several of the others contained MGF or GHRP-2 instead.
• All 9 that did contain follistatin contained a His-tag and significant oligomerization.
• The His-tag is what makes WADA detection unambiguous — it is never present on endogenous human follistatin, so any His-tagged follistatin in serum is unambiguous evidence of doping.

Bottom line: if you decide to use vendor follistatin, you have a roughly 50/50 chance the vial actually contains follistatin, your dosing route is not validated by any human trial, and you’re injecting a tagged recombinant protein with no published human PK or safety data.

Multiple Big Pharma programs have pursued myostatin inhibition (the same biology follistatin engages) and produced lean-mass gains without functional improvement. This is the most underused calibration point for follistatin claims.

• Domagrozumab (Pfizer, anti-myostatin antibody) — failed Phase 2 in DMD.
• Bimagrumab (Novartis, anti-ActRIIB antibody) — failed Phase 2 in sporadic IBM despite producing measurable lean-mass gains; functional outcomes did not improve.
• ACE-031 / ACE-083 (Acceleron, ActRIIB-Fc and follistatin-Fc fusion) — halted; cardiac/vascular AE signals.
• Landogrozumab (Lilly) — halted.
• Trevogrumab (Regeneron) — halted.

The repeating pattern: lean-mass gains are real, but functional improvement (strength, walking distance, daily-life function) does not consistently follow. Combined with cardiac/vascular AE signals across programs, the regulatory bar for myostatin pathway therapeutics has been very high — and pharma has mostly walked away.

Vendor claims of "20%+ muscle gain" from injected follistatin are not anchored in any published human trial, and the broader pharma class’s repeated functional-failure signal is the calibration the bodybuilding community typically ignores.

No human-trial-derived dose for injected follistatin protein exists. The Sarepta gene-therapy data uses AAV vector doses (vg/kg), not protein doses.

Community / vendor convention:

Parameter	Common Range
Dose per injection	50–100 µg SC
Frequency	Daily for 10 days, often repeated as cycles
Cycle length	10–30 days on, breaks of variable length
Route	SC; some IM near target muscle

None of these numbers is anchored in a published trial of injected follistatin protein. The Sarepta gene-therapy reference dose is ~6×10¹¹ vg/kg AAV1-FS344 IM into quadriceps — a fundamentally different intervention.

No published human safety data on injected follistatin protein. The gene-therapy safety record (PMIDs 25322757, 27858738, 28279643) is the closest thing — in BMD and sIBM patients receiving AAV1-FS344, no immune-mediated AEs were reported over follow-up.

Theoretical / class concerns:

• Cardiac and vascular signals — the failed Big Pharma anti-myostatin class produced cardiac/vascular AEs across multiple programs. Mechanism plausible: myostatin-pathway suppression in cardiac and vascular tissue.
• Activin A suppression downstream effects — activin A has roles in inflammation, wound healing, reproductive biology, and FSH regulation. Sustained follistatin elevation could disrupt these.
• FSH suppression — follistatin’s original-name function. FS288-style high-affinity binding is more disruptive than FS315; vendor "FST-344" identity confidence is low (per the Reichel 2019 forensics).
• Tendon / ligament weakness — rapid muscle gains outpacing connective-tissue adaptation is a class concern for any anabolic intervention.
• Cancer-axis — follistatin has been implicated in tumor biology; sustained follistatin elevation could plausibly support tumor growth via activin pathway suppression.
• Antibody response to His-tagged recombinant protein — not characterized for vendor "FST-344"; theoretical concern given the tag is non-native.

• Pregnancy / breastfeeding — absolute. Follistatin disrupts the activin-FSH axis.
• Active or prior cancer — class concern (myostatin-pathway suppression is bidirectional in cancer biology; activin A has tumor-suppressor roles in some lineages).
• Cardiovascular disease — class signal from failed pharma programs.
• Trying to conceive (female or male) — FSH disruption.
• Concurrent IGF-1 / HGH / SARM stack — uncharacterized; theoretical additive cardiac/vascular load.
• Athletes subject to anti-doping testing — do not use. WADA S2.1.4 names myostatin-function modifiers explicitly. The His-tag on vendor product is itself a doping fingerprint.

• ACE-031 — Acceleron ActRIIB-Fc fusion (myostatin pathway). Different molecule, similar pathway. Halted clinical program.
• ACE-083 — Acceleron follistatin-Fc fusion. Closer to follistatin biology; halted.
• Bimagrumab — Novartis anti-ActRIIB antibody. Different mechanism (receptor-blocking antibody vs ligand sequestration); failed Phase 2 in sIBM.
• YK-11 — SARM marketed as a "myostatin inhibitor" by some vendors. Mostly an AR agonist; the myostatin-inhibition mechanism is overstated in vendor copy.
• Epicatechin — flavonoid claimed to reduce myostatin via small-molecule mechanism. Limited human evidence; not equivalent to follistatin.
• Suprefort / Pancragen / other Khavinson peptides — unrelated short-peptide bioregulators; not myostatin pathway.
• SRP-9003 / AAV1-FS344 — Sarepta gene therapy, IM injection of AAV1 vector encoding the FS344 transgene. Different intervention from injected vendor "FST-344" protein. Don’t conflate.