FLGR-242
FLGR-242 (recombinant truncated follistatin construct)
Also known as: FLGR242, Recombinant follistatin, FS I-I variant
A gray-market recombinant follistatin fragment marketed since late 2025 as a myostatin trap that builds muscle; there are no published studies, no pharmacokinetics and no safety data on the molecule itself.
Overview
FLGR-242 is sold as a truncated follistatin protein with a claimed albumin-binding domain, at prices from a few hundred to several thousand dollars per vial. Vendors and the influencer who popularized it in November 2025 describe half a pound to a pound of muscle per week with most gains retained after stopping. The science behind the idea is real: follistatin binds myostatin and activins, and blocking myostatin increases muscle in animals and in one gene-therapy trial. But no paper characterizes FLGR-242, no study measures its half-life or effect, and a 2017 rebuttal questioned the efficacy claims made for follistatin gene therapy. Because follistatin also binds activin, which regulates FSH, reproductive suppression is a theoretical concern with no monitoring data.
Mechanism of action
Follistatin-type binding of myostatin and activins prevents them from engaging ActRIIA/ActRIIB receptors, releasing the Smad2/3 brake on muscle protein synthesis.
Key studies & citations
- Animal1997
Regulation of skeletal muscle mass in mice by a new TGF-beta superfamily member (myostatin)
Myostatin loss produces dramatic muscle growth in mice; the basis of the idea, not evidence for the product.
Nature - Human2015
A phase 1/2a follistatin gene therapy trial for Becker muscular dystrophy
Six patients; some distance-walked improvement; unrelated to FLGR-242.
Molecular Therapy - Review2017
Letter: questions on the follistatin gene therapy efficacy claims
Formal rebuttal of the reported efficacy.
Molecular Therapy
Frequently asked questions
Does FLGR-242 build muscle?
Nobody knows. There is no published study of FLGR-242 in animals or humans. The before-and-after photos circulating online are marketing, not data.
Is it safe?
Unknown. Follistatin also blocks activin, which helps regulate FSH and fertility, and there is no safety dataset of any kind. Treat it as the highest-risk entry in this database.
Is FLGR-242 the same as RTR242?
No. RTR242 is Retro Biosciences' oral autophagy drug in a regulated Phase 1 trial. The names look alike; the compounds are unrelated.