Follistatin

Follistatin is a human protein encoded by FST. The 344-amino-acid FST344 precursor is processed to the 315-amino-acid mature form; FST317 is a separate precursor isoform. These names describe biology, not interchangeable products. UniProt’s reviewed human entry records the 344-amino-acid precursor and its processing. The previous page’s 81-residue sequence did not represent FST344, so this review does not display it as a molecular identity.

What the mechanism shows

Follistatin binds members of the TGF-beta family, including activin and myostatin. A crystal-structure study shows two follistatin molecules surrounding activin A and covering receptor-binding surfaces. That structure does not measure clinical muscle benefit. In mice, follistatin transgene overexpression increased muscle mass even in a myostatin-null background, indicating effects beyond myostatin blockade. Other mouse experiments found that an intact IGF-1 receptor/Akt/mTOR pathway contributes to hypertrophy, but did not show that adding IGF-1 or an IGF-1 analogue improves outcomes with follistatin (IGF-1R study; IGF-I/insulin follow-up).

Human studies: three different interventions

AAV gene transfer. A phase 1/2a Becker muscular dystrophy study delivered a vector encoding FS344 to six people without a randomized control group. Four had improved six-minute-walk distance at reported follow-up and two did not. The small uncontrolled study is a signal, not proof of clinical effectiveness. A separate sporadic inclusion body myositis report compared six vector-treated participants, who also followed an exercise regimen, with eight matched untreated people. Its reported annualized walking difference cannot be attributed confidently to gene transfer because of design and sample-size limits; a published critique and author reply document the dispute. Neither study administered ordinary follistatin protein.

A separate Duchenne muscular dystrophy AAV1-FS344 registry reports posted results for an actual three-person, single-group phase 1/2 study. The registry reports 0 of 3 participants with increased six-minute-walk distance, 1 of 3 with North Star Ambulatory Assessment improvement, and 0 dose-limiting adverse events. Those mixed observations do not establish functional benefit or safety, especially at this sample size, and do not test administered native protein.

Engineered follistatin-based protein. ACE-083 is a locally acting recombinant fusion protein, not native FST315 and not AAV gene transfer. In a 58-person phase 1 placebo-controlled study, local muscle volume increased, but mean muscle strength did not significantly change. Later randomized phase 2 studies in facioscapulohumeral muscular dystrophy and Charcot-Marie-Tooth disease also found muscle-volume changes without consistent functional improvement. Their adverse-event observations and efficacy results cannot establish the safety or benefit of native follistatin preparations.

Native protein administration. The selected human intervention papers above do not test ordinary recombinant FST315 or FST288 as a treatment for muscle growth or wasting. Endogenous blood-level and exercise studies are not administration trials. It would be incorrect to count the AAV or ACE-083 cohorts as direct native-protein treatment evidence.

Preclinical and registry context

An AAV1-FS344 macaque study reported local muscle-size and strength changes. PubMed links a 2026 erratum to that paper; the accessible erratum record does not specify its correction, so this page does not rely on a precise quantitative macaque effect. In aged mice, AAV-mediated overexpression improved some muscle and neuromuscular-junction measures but did not prevent motor-unit loss. Neither model proves an anti-aging outcome in humans.

Other human registrations remain intervention-specific. NCT06411366 studies an FST344 plasmid gene-transfer approach, not injected protein. NCT07443826 describes an early open-label AAV9-follistatin safety study, with some cohorts also receiving a VEGF plasmid. NCT07285629 lists a follistatin plus klotho plasmid gene-transfer study as completed with actual enrollment of 14, but the 2026-09-28 registry snapshot has no posted results; any later outcome could not be attributed to follistatin alone. Registry status without posted results is not clinical outcome evidence. The FDA’s orphan designation record says the AAV-follistatin transgene has not been approved for its designated indication.

What remains unknown

• Whether native follistatin protein administration improves muscle strength, mobility, or health outcomes in people.
• Whether any observed muscle-volume change with an engineered analogue translates into dependable functional benefit.
• Long-term and product-specific safety of native protein, AAV gene transfer, and ACE-083; safety findings for one modality cannot be assigned to another.
• Effects or safety of combining follistatin with IGF-1 analogues, growth hormone, other myostatin inhibitors, BPC-157, or TB-500. Mechanistic differences do not establish compatibility or synergy.

This is an evidence summary, not medical advice. The selected source counts describe the records cited for this review, not all follistatin publications or independent clinical trials.