Follistatin 344

A full-length precursor isoform of the endogenous activin-binding glycoprotein follistatin. After signal peptide cleavage, the mature circulating isoform is FST315 (315 amino acids, ~38 kDa). Follistatin binds and neutralizes TGF-β superfamily ligands, primarily activin A/B and myostatin (GDF-8), preventing them from engaging the ActRIIB receptor and blocking downstream Smad2/3 signaling that suppresses muscle growth.

In transgenic mice, follistatin overexpression increased muscle mass 194-327% versus controls. However, the injectable recombinant protein has a very short circulating half-life (~1-2 hours in animals), meaning daily subcutaneous injections produce only transient biological effects. The dramatic muscle-building results cited in community discussions come from AAV gene therapy trials (sustained local expression for months), not from injectable protein.

Phase 1/2a gene therapy trials for Becker muscular dystrophy and inclusion body myositis showed promising functional outcomes, though these remain small studies with methodological limitations.

Follistatin's role in muscle biology is well-established through extensive animal research. Gilson et al. (2009) demonstrated follistatin induces muscle hypertrophy through both myostatin and activin inhibition plus satellite cell proliferation. Human data exists only for AAV gene therapy delivery: Mendell et al. (2015) showed improved 6-minute walk test in Becker muscular dystrophy patients, and a follow-up study (2017) reported improved ambulation over 3-5 years.

These gene therapy results cannot be extrapolated to injectable recombinant protein due to fundamental pharmacokinetic differences. Authoritative reviews note that injectable FS-344 would likely produce only transient myostatin inhibition with minimal impact on muscle mass. Community dosing protocols are not derived from clinical evidence.