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Evidence: LowProtein, not a peptidePreclinicalMyostatin blockerTested only as gene therapySold on the black market

Follistatin-344

The follistatin isoform behind one small gene-therapy trial in muscular dystrophy, and the vials sold online that often do not contain it

Follistatin binds myostatin, the signal that limits muscle growth. FS344 is the form used in the only trial in people, which delivered the gene by injecting a virus into six men's thigh muscles. Nobody has been given the protein itself in a study, and a test of black-market vials found only nine of 17 contained any follistatin.

At a Glance Read Research →
Overview

What is it?

Myostatin keeps muscle from growing past a set point, and follistatin is one of the body's own brakes on myostatin. Block myostatin and muscle grows, which is why follistatin interests both muscular dystrophy researchers and bodybuilders. FS344 is a spliced form chosen for gene therapy because it is less likely to bind sites outside muscle.

The one study in people was a 2015 phase 1/2a trial in Becker muscular dystrophy. Six men received an adeno-associated virus carrying the FS344 gene, injected directly into both quadriceps. Four walked further on the six-minute walk test, by 29 to 125 metres, two did not improve, and no adverse effects were reported; muscle biopsies showed less fibrosis and larger fibres.

That is gene therapy, made inside the patient's own muscle. It says nothing about injecting a vial of 'follistatin 344' bought online, which is what most people searching for it mean. A 2019 analysis of 17 such products found that only nine contained follistatin at all, all of them a His-tagged laboratory version; several contained other growth peptides instead.

Follistatin also circulates naturally, and in healthy people its levels fall after a glucose load, which has made it a candidate diabetes target as well as a muscle one. No approved follistatin product exists anywhere, and no study has measured what happens when the protein itself is injected into a person.

6
People in the only trial
gene therapy, Becker MD
9 of 17
Black-market vials containing it
2019 analysis
0
Studies of the protein in people
only the gene
Structured data

At a Glance

Every value in this table is printed from the Grey Peptides dataset, the same record the compound explorer and comparison tool read, so the three can never disagree.

ClassA 344-amino-acid splice variant of the follistatin protein that binds and neutralizes myostatin, activin A, and related TGF-β family ligands.
Molecular weight37871 g/mol
Half-lifeprotein; tissue-bound
Typical doseresearch-only (varies, intramuscular or subcutaneous (community))
US statusPreclinical
Evidence levelLow
Last reviewed2026-09-30
Evidence

Published Research

These are the studies that describe FS344, read from their PubMed abstracts: the gene therapy trial, the black-market analysis and a study of natural follistatin in people.

Human2015

Gene therapy in Becker MD

Six men received AAV1 carrying FS344 by injection into both quadriceps at two doses. Four improved on the six-minute walk by 58, 125, 108 and 29 metres; two did not. No adverse effects; biopsies showed less fibrosis and muscle hypertrophy.

PMID: 25322757
In Vitro2019

What is in the vials

17 black-market products labelled follistatin 344 or 315 were analysed. Only nine contained follistatin, all a His-tagged form with oligomers; others held growth peptides such as MGF and GHRP-2. A detection method for serum and urine was developed.

PMID: 31758732
Human2019

Natural follistatin and blood sugar

In 32 healthy people, circulating follistatin fell after glucose intake, and levels changed after bariatric surgery in 41 people with obesity, testing whether follistatins could be a diabetes target in humans.

PMID: 30393997
Safety

Side Effects & Contraindications

Reported Side Effects

From the gene therapy trial; the protein has no human safety record.

● No adverse effects in the six-patient gene therapy trial
● Gene therapy cannot be stopped once given
● Black-market products may contain other peptides than labelled, per the 2019 analysis
● No safety data exist for injected follistatin protein

Contraindications & Cautions

No approved product exists.

● Self-injection of products sold as follistatin 344 — no human study of the protein exists
● Reading the gene therapy result as evidence for injected protein
● Tested athletes — see the Sport (WADA) row below
Questions

Frequently Asked Questions

?What is follistatin 344?

A 344-amino-acid form of follistatin, a protein that blocks myostatin, the signal limiting muscle growth.

?Has it been tested in people?

Only as gene therapy: six men with Becker muscular dystrophy received the gene by injection into their thighs, and four walked further.

?Do online vials contain it?

Often not. A 2019 analysis found follistatin in only nine of 17 black-market products, and other peptides in some.

?Is it allowed in sport?

No. WADA prohibits myostatin inhibitors including follistatin.

Bibliography

References

  1. [pubmed] Mendell JR, et al. "A phase 1/2a follistatin gene therapy trial for becker muscular dystrophy." Mol Ther, 2015;23(1):192-201. PMID: 25322757.
  2. [pubmed] Reichel C, et al. "Detection of black market follistatin 344." Drug Test Anal, 2019;11(11-12):1675-1697. PMID: 31758732.
  3. [pubmed] Perakakis N, et al. "Follistatins in glucose regulation in healthy and obese individuals." Diabetes Obes Metab, 2019;21(3):683-690. PMID: 30393997.
  4. [pubmed] Lee SJ. "Regulation of muscle mass by myostatin." Annu Rev Cell Dev Biol, 2004;20:61-86. PMID: 15473835.
  5. [clinicaltrials] ClinicalTrials.gov NCT01519349: follistatin gene transfer to patients with Becker muscular dystrophy and sporadic inclusion body myositis, completed (registry record read September 30, 2026).

Sources & Citations

Every study here was read from its PubMed record.

Medical Disclaimer: This content is for educational and informational purposes only. It is not medical advice, and nothing here is an instruction to obtain or use any compound. Consult a licensed healthcare provider before making health decisions.

Last reviewed: 2026-09-30