Growth ⏱ Half-life: Tissue-bound — longer local duration than circulating FS344 but less systemic persistence. Experimental

Follistatin 315

Follistatin 315

Updated March 15, 2026

Evidence at a Glance

Regulatory statusExperimental
Mature circulating follistatin isoform; sold as a recombinant protein with no human trials of its own; related FS344 gene therapy reached Phase 1/2a
Human evidenceNone
Animal evidenceLimited

FST-315 is the naturally circulating follistatin isoform, but injecting the recombinant protein for muscle growth has never been tested in a human trial and rests on animal and gene-therapy extrapolation.

What the research does not support

  • It is not FDA-approved and injecting this recombinant protein has never been studied in a human clinical trial.
  • The human muscle data attributed to follistatin come from AAV gene therapy (FS344), a different modality, not from injecting this protein.
  • It does not cleanly block only myostatin — it also binds activins and other TGF-beta ligands, with unknown systemic effects.
  • Claims of large, lasting muscle gains in people from injectable follistatin are unproven.
Half-Life
Tissue-bound — longer local duration than circulating FS344 but less systemic persistence.
Mol. Weight
~35,000 g/mol (glycoprotein)

What is Follistatin 315?

Follistatin 315 is the tissue-selective isoform of follistatin that remains bound to cell surfaces rather than circulating freely. This localization provides myostatin-inhibiting effects with a more targeted profile than the systemic Follistatin 344 variant.

Research Applications

Localized myostatin inhibition, muscle-specific growth signaling, muscular dystrophy gene therapy research, and reduced off-target endocrine effects vs. FS344.

Dosage Information (Research Use)

Research protocols: 100-300 mcg/day subcutaneously. Similar cycling to FS344. Research use only.

Reconstitution & Handling

Reconstitute carefully with BAC water. Gentle handling — do not shake.

Half-Life & Pharmacokinetics

Tissue-bound — longer local duration than circulating FS344 but less systemic persistence.

Reported Observations in Literature

Theoretically reduced reproductive hormone effects compared to FS344 due to tissue localization. Limited human safety data.

Key Research References

  • Kota J, et al. “Follistatin gene delivery enhances muscle growth and strength in nonhuman primates.” Sci Transl Med. 2009

How Follistatin 315 Works

Follistatin 315 (FS315) is the tissue-bound isoform of follistatin, as opposed to the circulating FS344 form. FS315 binds to cell surface heparan sulfate proteoglycans, keeping it localized to muscle and other tissues rather than circulating systemically. It still neutralizes myostatin and activin, but with more localized, tissue-specific effects. This localization reduces potential systemic effects on reproductive hormones (activin is involved in FSH regulation), making FS315 theoretically more targeted than FS344.

Research Findings

Preferred in some research protocols over FS344 due to reduced potential for systemic endocrine disruption. More stable in solution than FS344. Gene therapy approaches for muscular dystrophy have specifically used FS315-based constructs.

Dosage & Administration

Research protocols: 100-300 mcg/day subcutaneously. Similar cycling to FS344. Research use only.

Safety & Side Effects

Theoretically reduced reproductive hormone effects compared to FS344 due to tissue localization. Limited human safety data.

Important: All safety information is derived from published research, primarily animal studies. No controlled human clinical trial data exists unless explicitly noted. This compound is sold for research purposes only.

Quick Facts

Sequence 315-amino acid glycoprotein (shorter isoform)
Molecular Weight ~35,000 g/mol (glycoprotein)
Half-Life Tissue-bound — longer local duration than circulating FS344 but less systemic persistence.
Available Sizes 1mg
Storage Lyophilized: -20°C. Reconstituted: 2-8°C, use within 14 days.

Key Research References

  • Kota J, et al. "Follistatin gene delivery enhances muscle growth and strength in nonhuman primates." Sci Transl Med. 2009

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