RESEARCH MONOGRAPH · KDC-MN-090

Follistatin 315

May 9, 2026 Kodiac biolabs Research Revised May 30, 2026 2 min read

Our opinion on this compound

49/100

The longer follistatin isoform with C-terminal acidic tail, similar biology to FS-344 with even worse sourcing reality

Follistatin 315 (FS-315) is the longer of the two principal follistatin isoforms, arising from alternative splicing of the FST gene to retain an additional C-terminal acidic domain. Functionally it binds the same TGF-beta superfamily ligands (myostatin, activin A, GDF-11) with comparable affinity to FS-344, but the C-terminal acidic tail confers different heparan sulfate proteoglycan binding properties. FS-315 has lower cell-surface association and is more freely circulating, while FS-344 tends to accumulate at extracellular matrix surfaces.

What we like is the structural pharmacology question. The two isoforms give researchers a tool to disentangle cell-surface vs. circulating follistatin biology, which matters for understanding tissue-specific myostatin neutralization. The Welt group and the Acceleron researchers have done some work on isoform-specific effects in different model systems, and the structural biology (Innis 2014 and earlier crystallography work) is reasonably well-mapped.

What we dont like: essentially the same issues as FS-344 but with smaller literature support. The clinical relevance of FS-315 vs. FS-344 specifically is mostly speculative, since approved myostatin-pathway drugs use receptor decoy approaches rather than follistatin protein. The Acceleron and Keros programs validated the broader TGF-beta superfamily target class but didn't specifically validate follistatin protein therapy. So the human translation story for FS-315 specifically is even thinner than for FS-344.

Sourcing problems are essentially identical to FS-344. The protein requires proper folding, has multiple disulfide bonds, and needs mammalian or insect cell expression for activity. Bacterial expression products are inactive. Most commercial preparations in the research peptide market lack the analytical packages needed to verify correct folding, glycosylation, and bioactivity. R&D Systems and Peprotech grade material is real; cheap supplier material is mostly not what the label says.

For mechanistic research where the isoform-specific differences in heparan sulfate binding or matrix association are relevant, FS-315 has a defined research niche. For everything else, the FS-344 vs. FS-315 question is mostly academic and the sourcing limitations make the practical research utility severely constrained. Treat it as a specialized tool, not a working pharmacology compound.

Evidence: 45
Mechanism: 72
Reliability: 48
Safety: 55
Sourcing: 32
Value: 38
Signal: 50
Staying power: 50

Plain-language summary Intrigue 70 / 100

Follistatin-315 is the 315-amino-acid isoform of follistatin, which differs from FST-344 by tissue distribution and binding partners. Same general myostatin-blocking function. Not stocked by Kodiac. This monograph is provided for research and educational reference.

Intrigue 0–100 blends mechanism novelty, evidence strength, and translational potential. Kodiac editorial, not peer-reviewed.

Myostatin antagonist / follistatin isoform

A 315-residue isoform of follistatin with reduced heparin binding and longer plasma half-life than follistatin 344.

Abstract

Follistatin 315 is a 315-residue isoform of follistatin produced by alternative splicing of the FST gene, lacking the C-terminal acidic region present in follistatin 344. The compound has reduced heparin binding (which improves plasma circulation; follistatin 344 is largely tissue-bound through cell surface heparan sulfate) and a longer plasma half-life. Like follistatin 344, the compound antagonizes myostatin (GDF-8) and activin A. The compound is sold as a research peptide. There is no formal clinical development record. Investigational doses parallel follistatin 344; safety considerations are similar.

Mechanism of action

Endogenous myostatin/activin antagonist; longer half-life than follistatin 344.

Reported research dose ranges

100 micrograms to 1 mg SC/IM (reported research dose ranges in the literature).

References

Welt C, et al. The follistatin family. Endocrinology 2006.

Read the full monograph

The full reference document covers compound identification, discovery and developmental history, mechanism of action, pharmacokinetics, reported research dose ranges, sourcing and quality verification, reconstitution and handling, stack interaction considerations, and a curated reference list. Available as a research-use-only PDF download.

KDC-MN-090

The full reference document is provided strictly for research use only. It reports research dose ranges from the published literature, not instructions for use in humans or animals.

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FOR RESEARCH USE ONLY. Not for medical, diagnostic, or therapeutic purposes. Not for human consumption. All information is provided for research and educational purposes only.

Kodiac sells research compounds for investigative use only.

Follistatin 315

Abstract

Mechanism of action

Reported research dose ranges

References

Read the full monograph

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Abstract

Mechanism of action

Reported research dose ranges

References

Read the full monograph

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