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. It's 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 get sold and discussed, and they're not interchangeable.

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.

Some encyclopedia content lists isoforms as "FS288, FS303, FS315." That's wrong — the standard modern triumvirate is FS288, FS315, FS344. FS303 isn't the standard third name.

Regulatory status

• FDA, EMA: not approved. Multiple INDs have existed (Acceleron's ACE-083, Sarepta/Nationwide's AAV1-FS344 / SRP-9003) — none have produced an approved product.
• Not on the FDA 503A bulks list. Sold as research chemical only.
• WADA 2026: prohibited at all times under S2.1.4 (myostatin function modifiers) — a specific WADA classification with dedicated detection methodology, not just S0 catch-all.
• IFHA (horseracing) Article 6 prohibition; NCAA, NFL, MLB banned class.

Follistatin binds and sequesters TGF-β-superfamily ligands, preventing them from engaging their type I and type II receptors. The primary targets are activins A and B (the historical target — Thompson et al. Dev Cell 2005, PMID 16198295, solved the crystal structure showing two follistatin molecules wrapping a single activin dimer and blocking both type I and type II receptor binding) and myostatin / GDF-8 (Cash et al. EMBO J 2009, PMID 19644449, solved the myostatin:FS288 complex with similar two-follistatin "necklace" topology). BMP-2, BMP-4, and BMP-7 are bound more weakly.

Downstream, myostatin normally signals via ActRIIB → ALK4/5 → SMAD2/3 phosphorylation, suppressing muscle protein synthesis. Follistatin sequestration reduces SMAD2/3 phosphorylation, derepressing muscle protein synthesis. Net effect: increased muscle mass and lean tissue, reduced fibrosis in some models.

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

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

The real clinical evidence comes from the Mendell / Nationwide Children's / Sarepta program — gene therapy via AAV1-FS344 vector injected directly into muscle. The patient's own muscle then manufactures FS344 protein locally and secretes it as the FS315 serum form. Mendell 2015 (PMID 25322757) was the first-in-human IM injection in n=6 Becker muscular dystrophy patients — quadriceps injection, ~6×10¹¹ vg/kg dose — showing 4/6 with improved 6-minute walk test, reduced fibrosis on biopsy, and no immune-mediated AEs. PMID 27858738 (2015) reviewed the FS344-vs-FS288 isoform rationale and reported sustained ambulation improvements. PMID 28279643 (2017) extended the program to n=6 sporadic IBM patients with mixed but generally encouraging functional outcomes. The clinical program now markets as SRP-9003 under Sarepta, with Phase 3 in progress (advanced into LGMD-2E rather than primarily BMD).

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

Vendor "FST-344" is a different intervention. Black-market 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 had a His-tag and significant oligomerization. The His-tag is what makes WADA detection unambiguous — it's 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 isn't 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, with functional outcomes that didn't improve. ACE-031 and ACE-083 (Acceleron, ActRIIB-Fc and follistatin-Fc fusion) were halted with 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) doesn't 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 aren't 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.

Parameter	Community 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.

Class concerns to know about: cardiac and vascular signals from the failed Big Pharma anti-myostatin class (mechanism plausibly attributable to 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, and sustained follistatin elevation could disrupt these); FSH suppression — follistatin's original-name function (FS288-style high-affinity binding is more disruptive than FS315, and vendor "FST-344" identity confidence is low per the Reichel 2019 forensics); tendon and ligament weakness from rapid muscle gains outpacing connective-tissue adaptation; a cancer-axis concern (follistatin is implicated in tumor biology, and sustained elevation could plausibly support tumor growth via activin-pathway suppression); and antibody response to the His-tagged recombinant protein in vendor product, which isn't characterized but is theoretically concerning given the tag is non-native.

• Pregnancy or breastfeeding — absolute. Follistatin disrupts the activin-FSH axis.
• Active or prior cancer — class concern (bidirectional in cancer biology; activin A has tumor-suppressor roles in some lineages).
• Cardiovascular disease — class signal from failed pharma programs.
• Trying to conceive (male or female) — 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.

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.