LL-37: The Human Cathelicidin Peptide, From a Predicted 39-mer to a Defined 37-Residue Reagent

Research use only: the materials discussed here are sold for laboratory research. They are not approved drugs or supplements and are not for human or veterinary use.

Research context: this overview summarises published laboratory literature for researchers. It contains no guidance on human use, dosing or preparation for administration.

Disclosure: this overview was prepared on behalf of Peptide Titans, a research-peptide supplier, and links once to its product page.

Short answer: LL-37 is the 37-residue C-terminal peptide of human cathelicidin. Its sequence is LLGDFFRKSKEKIGKEFKRIVQRIKDFLRNLVPRTES. It was first predicted as a slightly longer peptide, FALL-39, and the mature form isolated from granulocytes was then shown to begin at the two leucine residues that give the reagent its name. The synthetic 37-mer is a well-defined laboratory peptide with a large biochemical literature, much of it on membrane interaction and in-vitro antimicrobial assays in defined media. That literature describes a host-defence peptide, not an approved drug. For a methods section, the useful facts are the sequence, the expected mass and the lot certificate. Nothing in this overview is a basis for human use.

What "LL-37" actually names

The letters are the first two residues. The peptide is a single, unmodified 37-mer with a free N-terminus and a free C-terminal acid, derived from the human cathelicidin precursor (the gene originally discussed as FALL39, later often referred to as CAMP). It is amphipathic and, in many descriptions, helical in membrane-like environments. It contains multiple lysine and arginine residues, so it is strongly cationic at typical laboratory pH.

The specifications on Peptide Titans' LL-37 listing describe that sequence, formula C₂₀₅H₃₄₀N₆₀O₅₃ and an expected mass of about 4,493.3 Da, supplied as a white to off-white lyophilised powder in 5 mg vials, with a lot-specific certificate of analysis reporting HPLC purity and mass-spectrometry identity. The page states that the material is sold for laboratory research and is not an FDA-approved drug or a dietary supplement.

Two practical points follow:

How the peptide was identified

In 1995, Agerberth, Gunne, Odeberg, Kogner, Boman and Gudmundsson reported in the Proceedings of the National Academy of Sciences (PMID 7529412; doi 10.1073/pnas.92.1.195) a cDNA from a human bone-marrow library that encoded a cathelin-like precursor and a predicted 39-residue, cysteine-free peptide they named FALL-39 after its first four residues. They chemically synthesised the predicted peptide and reported activity against Escherichia coli and Bacillus megaterium in basal medium E. Circular dichroism indicated helix formation in that medium, and RNA blot analysis detected expression mainly in bone marrow and testis. That paper is the origin of the predicted 39-mer. It is an in-vitro characterisation, not a clinical study.

The following year, Gudmundsson, Agerberth, Odeberg, Bergman, Olsson and Salcedo published the gene structure in the European Journal of Biochemistry (PMID 8681941; doi 10.1111/j.1432-1033.1996.0325z.x). They described a compact four-exon gene, with exons 1–3 encoding the signal sequence and cathelin region and exon 4 encoding the mature antimicrobial peptide. Using antibodies, they located the peptide in granulocytes and isolated the mature peptide from those cells after degranulation. Structural analysis showed the mature peptide to be LL-37, not FA-LL-37. To obtain material for antibacterial assays, they degraded synthetic FA-LL-37 with dipeptidyl-peptidase I and reported that mature LL-37 was a potent antibacterial peptide in their assays.

That is the key identity paper: the isolated mature form is the 37-mer. The antibacterial results are in-vitro assays. They do not establish that a commercial research vial treats infection in people, and they should not be written up as if they did.

Later biophysical work, and what it does not decide

Dürr, Sudheendra and Ramamoorthy reviewed the peptide in Biochimica et Biophysica Acta in 2006 (PMID 16716248; doi 10.1016/j.bbamem.2006.03.030), under the title "LL-37, the only human member of the cathelicidin family of antimicrobial peptides." They summarised structural and biophysical evidence for a 37-residue, amphipathic, helical peptide, noted a broad range of reported in-vitro antimicrobial activity, and outlined additional reported laboratory effects on immune cells and membranes. The review is useful as a map of what had been measured by 2006. It is not, on its own, confirmation of each reported activity, and it is not a clinical monograph.

LL-37 also appears in many later studies as an analyte, including work in which the peptide is measured in human samples. A ClinicalTrials.gov search we ran on 12 November 2026 for "LL-37" returned several dozen registered records. Inspection of examples shows that the name often appears as a measured marker rather than as the study treatment. A registry entry is not a result, and a measurement in a trial is not an approved indication for a synthetic research peptide.

What a laboratory protocol actually needs

At 37 residues and with a high density of basic residues, the peptide can adsorb to surfaces and can aggregate or change conformation depending on buffer, salt and the presence of membranes or LPS-like molecules. Those are experimental-design issues, not use instructions. A mass spectrum should be compared with the expected mass of about 4,493.3 Da. Because the chain is long enough for deletion peptides and truncated impurities to be plausible, a chromatogram with a stated method matters as much as a single mass peak.

General peptide-handling guidance applies. Bachem's published handling notes observe that peptides tend to be hygroscopic, and they recommend allowing a sealed container to reach ambient temperature in a dry atmosphere before opening, then weighing quickly and resealing. They also recommend aliquoting solutions and keeping them frozen, and they advise against long-term storage in solution. The Peptide Titans listing specifies −20 °C to −80 °C for long-term storage of the sealed lyophilised vial, protected from light and moisture, with 2–8 °C as a short-term option.

A useful certificate should report an identity result against the expected mass of this 37-mer, a purity figure whose method is stated, and a lot number that matches the vial. Results from one lot should not be generalised to another.

Bottom line

LL-37 is a defined 37-residue human cathelicidin peptide with a clear discovery path from a predicted 39-mer to a mature form isolated from granulocytes. The laboratory literature on structure and in-vitro antimicrobial assays is substantial. That record makes the synthetic peptide a specified research reagent. It does not make it a medicine, and it is not a basis for human use. If you work with it, state the sequence, confirm the mass, record the lot, and keep the interpretation inside the experiment you actually ran.

Research use only: products discussed in this article are sold strictly for laboratory research. They are not for human or veterinary use and are not intended to diagnose, treat, cure, or prevent any disease. Product details were read from the supplier's listing as captured in October–November 2026 (Ahrefs Site Audit crawl of 11 November 2026, which recorded the product URL as HTTP 200, indexable and self-canonical, together with the store catalog record of 23 October 2026 for the listed vial size). Details may change.

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