Is follistatin a peptide?

Not in the usual sense. Follistatin is a glycoprotein: its main mature form has 315 amino acids, 18 disulfide bonds and sugars attached at two sites. It is sold alongside short peptides, but it needs protein-style testing such as peptide mapping, SDS-PAGE and protein mass spectrometry rather than a single intact mass check.

What is the difference between follistatin 344 and follistatin 315?

They usually describe the same protein at different stages. UniProt lists a 344-residue precursor with a 29-residue signal peptide, which leaves a 315-residue mature chain. Follistatin 288 is a different isoform that lacks the last 27 residues of the precursor. A test report should say which form it detected, and whether it carries an added tag.

Why can't a certificate give one exact mass for follistatin?

Because the protein carries N-linked sugars at two sites, and ICH guidance notes that proteins made by living organisms can be mixtures of glycoforms. UniProt's 38,007 Da figure covers only the unglycosylated 344-residue precursor chain, not the mature glycosylated protein. A report should describe the glycoform spread or confirm identity by peptide mapping instead.

One of the 30 compound testing profiles

Follistatin

Follistatin is a glycoprotein, not a short peptide: its main mature chain has 315 amino acids and 18 disulfide bonds, and in a 2019 anti-doping study only nine of 17 black-market products contained it at all, so a test has to confirm the protein is present before anything else.

Follistatin is a large, cysteine-rich human glycoprotein that UniProt records as a 344-residue precursor with a 29-residue signal peptide, 18 disulfide bonds and two N-linked glycosylation sites (UniProt P19883).

Test your follistatin

Identity card

Formula: Not a single molecule (see note)
Average mass: Not published
Monoisotopic mass: Not published
Length: 315 residues
Modifications: Signal peptide (precursor residues 1 to 29) removed in the mature chain; 18 disulfide bonds; N-linked glycosylation at Asn124 and Asn288 (precursor numbering)
Also sold as: Follistatin 344, FS344, Follistatin 315, FS315, FS288, FST
Sequence: GNCWLRQAKNGRCQVLYKTELSKEECCSTGRLSTSWTEEDVNDNTLFKWMIFNGGAPNCIPCKETCENVDCGPGKKCRMNKKNKPRCVCAPDCSNITWKGPVCGLDGKTYRNECALLKARCKEQPELEVQYQGRCKKTCRDVFCPGSSTCVVDQTNNAYCVTCNRICPEPASSEQYLCGNDGVTYSSACHLRKATCLLGRSIGLAYEGKCIKAKSCEDIQCTGGKKCLWDFKVGRGRCSLCDELCPDSKSDEPVCASDNATYASECAMKEAACSSGVLLEVKHSGSCNSISEDTEEEEEDEDQDYSFPISSILEW
Note: Follistatin is a protein, so it has no PubChem compound record and no single formula. The sequence shown is the mature chain of isoform 1 (FS315), residues 30 to 344 of UniProt P19883. UniProt gives 38,007 Da for the 344-residue precursor polypeptide without glycans; isoform 2 (FS288) has a 317-residue precursor of 34,803 Da. Glycosylation adds variable mass, so no single average or monoisotopic mass applies to the real protein.

Source: Follistatin (P19883), human.

Compare the mass printed on a certificate with the values above. The mass checker explains charge states and which mass a lab usually reports.

What usually goes wrong with follistatin. Documented failure modes, with sources.

Vials that contain no follistatin

Anti-doping researchers tested 17 black-market products, most labelled follistatin 344 and a few follistatin 315. Only nine contained follistatin; in some of the others, growth-promoting peptides such as MGF and GHRP-2 were found instead (Reichel et al., 2019). That is an identity failure, so a follistatin test has to use a method that detects a large glycoprotein, not one tuned only to small peptides.

What 344, 315 and 288 mean

UniProt describes a 344-residue precursor whose first 29 residues form a signal peptide, leaving a 315-residue mature chain known as FS315; a second isoform, FS288, comes from a 317-residue precursor that lacks the last 27 residues (UniProt P19883). FS288 and FS315 are splice variants that differ by a C-terminal extension, which changes how they bind heparin (Lerch et al., 2007). A label reading 344 names the precursor; 315 and 288 name mature forms. A report should say which form it found.

Added His-tags and oligomers

All nine products in that study that did contain follistatin held His-tagged FS344 with a high degree of its oligomers, and the tag allowed unambiguous differentiation from the body's own follistatin (Reichel et al., 2019). A His-tag changes sequence and mass compared with natural follistatin. ICH guidance treats aggregates, dimers and higher multiples, as product-related impurities to be resolved and quantified, for example by size exclusion chromatography (ICH Q6B).

Thirty-six cysteines and variable sugars

The mature FS315 chain contains 36 cysteines joined in 18 disulfide bonds, with N-linked sugars at two asparagines (UniProt P19883). Proteins with high cysteine content and complex disulfide connectivity are a named challenge for mass spectrometric disulfide analysis, as is preventing disulfide scrambling during sample preparation (Lakbub et al., 2018). ICH guidance adds that proteins made by living organisms can be mixtures of glycoforms (ICH Q6B), so intact mass is a spread, not a single number.

Which tests matter most

Peptide mappingCutting the protein into fragments and identifying them confirms the follistatin sequence and reveals a His-tag; ICH notes peptide mapping is frequently used to confirm protein structure.
LC-MSIdentifies digest fragments and, for intact protein, shows the spread of glycoforms rather than a single mass.
MALDI-TOFGives an approximate intact mass for a large glycoprotein, enough to tell follistatin from the small peptides found in mislabelled vials.
Amino acid analysisCan measure protein content, although ICH notes composition data are less definitive for large proteins.

Reading follistatin results

Peptide identity testing compares one measured mass with one expected mass, which does not transfer cleanly to follistatin. ICH guidance for proteins lists amino acid sequence, terminal sequence, a peptide map, disulfide bridge positions, carbohydrate structure, and molecular weight by size exclusion chromatography, SDS-PAGE or mass spectrometry (ICH Q6B). The anti-doping study detected black-market follistatin by SDS-PAGE and Western blotting with antibodies against the His-tag or follistatin itself (Reichel et al., 2019).

Reichel and colleagues noted in 2019 that no approved pharmaceutical formulation of follistatin was available, and that follistatin is prohibited under the World Anti-Doping Agency list (Reichel et al., 2019). Products sold under the name are therefore made to no published specification, and the identity method has to be agreed with the partner laboratory before testing.

Public Tested.ph results

No public follistatin records yet

Each public follistatin record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing follistatin

Is follistatin a peptide?: Not in the usual sense. Follistatin is a glycoprotein: its main mature form has 315 amino acids, 18 disulfide bonds and sugars attached at two sites. It is sold alongside short peptides, but it needs protein-style testing such as peptide mapping, SDS-PAGE and protein mass spectrometry rather than a single intact mass check.
What is the difference between follistatin 344 and follistatin 315?: They usually describe the same protein at different stages. UniProt lists a 344-residue precursor with a 29-residue signal peptide, which leaves a 315-residue mature chain. Follistatin 288 is a different isoform that lacks the last 27 residues of the precursor. A test report should say which form it detected, and whether it carries an added tag.
Why can't a certificate give one exact mass for follistatin?: Because the protein carries N-linked sugars at two sites, and ICH guidance notes that proteins made by living organisms can be mixtures of glycoforms. UniProt's 38,007 Da figure covers only the unglycosylated 344-residue precursor chain, not the mature glycosylated protein. A report should describe the glycoform spread or confirm identity by peptide mapping instead.

Sources

Follistatin (P19883), human, UniProt
Reichel C, Gmeiner G, Thevis M. Detection of black market follistatin 344. Drug Test Anal. 2019;11(11-12):1675-1697 (erratum: Drug Test Anal. 2020;12(10):1522-1533), Drug Testing and Analysis (PubMed)
Lerch TF et al. Structural and biophysical coupling of heparin and activin binding to follistatin isoform functions. J Biol Chem. 2007;282(21):15930-9, Journal of Biological Chemistry (PubMed)
ICH Q6B: Specifications, test procedures and acceptance criteria for biotechnological/biological products, International Council for Harmonisation
Lakbub JC et al. Recent mass spectrometry-based techniques and considerations for disulfide bond characterization in proteins. Anal Bioanal Chem. 2018;410(10):2467-2484, Analytical and Bioanalytical Chemistry (PMC)

Last reviewed 6 Oct 2026.