Why do MC1R variants cause red hair?

Loss-of-function variants leave the receptor signalling poorly, shifting production toward pheomelanin, which is red to yellow.

Does the receptor do anything else?

It has been associated with other melanocyte processes, which is what the 'and beyond' in the 2001 review's title refers to.

MT-2 (Melanotan II)

The Receptor That Decides Which Pigment Is Made

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-233 cited sources

The melanocortin-1 receptor governs which type of melanin a melanocyte produces — eumelanin or pheomelanin — rather than simply how much. Schaffer and Bolognia reviewed it in Archives of Dermatology in 2001 under the title red hair and beyond.

Key facts

Receptor: MC1R, on melanocytes
Governs: Which melanin type is produced
Two pigment types: Eumelanin and pheomelanin
Loss-of-function variants: Associated with red hair
Review: Schaffer 2001 (PMID 11708951)
Recent work: Berns 2024 (PMID 37972124)

Two pigments, not one

Melanocytes make two chemically distinct pigments. Eumelanin is brown to black and absorbs ultraviolet effectively. Pheomelanin is red to yellow and does not. Which one a cell produces is a switch rather than a dial, and MC1R signalling is what sets it.

What the receptor actually does

Active MC1R signalling shifts production toward eumelanin. Reduced signalling shifts it toward pheomelanin. So the receptor is not a volume control on pigment quantity — it is a selector between two products with different properties, and the difference in ultraviolet absorption is why the distinction matters biologically.

Research material referenced

MT-2 10mg — third-party HPLC tested

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Why variants produce red hair

Loss-of-function variants in MC1R leave the receptor signalling poorly, so the switch sits toward pheomelanin. The resulting phenotype is the red hair and fair skin association the Schaffer review is named for. It is a receptor-function phenotype, which is why it is inherited in the pattern it is.

The and beyond in that title

MC1R signalling has been associated with processes other than pigment selection, including aspects of DNA damage response in melanocytes. Berns and colleagues published single-cell profiling of MC1R-inhibited melanocytes in Pigment Cell and Melanoma Research in 2024, which is the kind of work that resolves what else the receptor is doing at cell level.

Why this is receptor biology rather than a cosmetic topic

The question here is what a receptor does to a cell's biosynthetic output and what else changes when it is engaged or inhibited. That is pharmacology, and it is the reason MC1R appears in melanoma research as well as in pigmentation genetics.

The position on this site

This describes published receptor biology. MT-2 holds no marketing authorisation anywhere, this category carries no appearance or cosmetic framing of any kind, and nothing supplied here is for use by any person.

Extended research context

The MT-2 (Melanotan II) deep dive

Deep dive: the identifier collision that points the wrong way

This site has now found several plausible-looking identifiers that resolve to the wrong molecule. A PubChem record for tirzepatide cited as retatrutide. Thymosin beta-4's CAS quoted as TB-500's. An unrelated organic acid returned for KPV. This one is the most consequential of the set, and the reason is direction. Searching PubChem for melanotan returns CID 16197727 - afamelanotide, at 1,646.8 Da for C78H111N21O19. That is Melanotan I, a linear thirteen-residue peptide, and it is an APPROVED MEDICINE, marketed as Scenesse for erythropoietic protoporphyria. Melanotan II is CID 92432: cyclic, seven residues, 1024.2 Da, 622 Da lighter, and assessed by no regulator anywhere. So the wrong record does not merely mislead about mass. It resolves from an unlicensed compound toward a licensed one, and the sequential naming - I and II - makes them look like versions of a single product rather than the structurally distinct compounds they are. Anyone verifying a certificate by searching the bare name lands on an approved drug's record and may never notice which compound it names.

Deep dive: four stability features in seven residues

PubChem's IUPAC name for CID 92432 is unusually complete and reads as a full specification: N-acetyl-L-norleucyl-L-alpha-aspartyl-L-histidyl-D-phenylalanyl-L-arginyl-L-tryptophyl-L-lysinamide, cyclic (2-7)-peptide. Unpack it and there are four independent protease-resistance features in a molecule of seven residues. A lactam bridge joins the aspartate side chain at position 2 to the lysine side chain at position 7, closing the ring - which both locks conformation for receptor engagement and eliminates the free termini exopeptidases require. Position 4 is D-phenylalanine, the mirror image of the natural form, and proteases are stereospecific enough that they largely cannot process it. The N-terminus is acetylated and the C-terminus amidated, capping both ends. And position 1 is norleucine, which is not among the twenty proteinogenic amino acids at all - so even setting aside the cyclisation and the D residue, this peptide could never have been a gene product. It is a fully synthetic construction rather than a modified natural sequence, and it is among the most chemically robust compounds in this catalogue as a direct result.

Deep dive: why the safety literature is included rather than omitted

There is a published dermatological literature here, and the editorial choice was to state it. Reid and colleagues documented atypical melanocytic naevi following melanotan injection in the Irish Medical Journal in 2013. Eijmael and colleagues published a risk review in the Nederlands Tijdschrift voor Geneeskunde in 2022 - a national medical journal considering the topic worth addressing directly. Case reports have a real limitation: they establish an observation without a denominator, so they cannot establish causation or incidence, which is the same constraint that applies to pharmacovigilance disproportionality signals. But that limitation cuts both ways. A case report is not proof of harm and it is not dismissible either. What makes the absence of better data significant here is that no regulator has assessed this compound - so there is no label, no contraindications, no adverse event reporting requirement and no monitoring framework. The information that would normally exist for an injectable product simply does not, and what is available instead is what individual clinicians chose to publish after seeing something. Presenting the compound without that context would give an incomplete picture of what is actually known.

Research applications

▸Melanocortin receptor pharmacology and selectivity research
▸Cyclic peptide and lactam bridge design studies
▸D-amino acid substitution and protease resistance research
▸Structure-activity work on constrained peptide analogues
▸Comparative work on alpha-MSH derivatives
▸Analytical method development for cyclic peptides

Handling checklist

✓Verify against CID 92432, 1024.2 Da, C50H69N15O9, CAS 121062-08-6
✓Never verify by searching 'melanotan' alone - that returns afamelanotide at 1,646.8 Da
✓Require stereochemistry on the certificate - D-Phe4 is not detectable by mass
✓Protect from light; tryptophan at position 6 is photochemically reactive
✓Store lyophilised, cold, dry and dark
✓Expect no disulfide or oxidation satellites - no cysteine, no methionine
✓Quantification at 280 nm is available thanks to the tryptophan

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Searching PubChem for 'melanotan' to verify identity

Fix: That returns CID 16197727, afamelanotide, at 1,646.8 Da - an approved medicine and a different molecule. Use CID 92432 or search Melanotan II with the numeral.

✗ Treating Melanotan I and II as versions of one compound

Fix: They differ by 622 Da, by six residues, and by whether the peptide is cyclic or linear. Only Melanotan I has been assessed by regulators.

✗ Reading afamelanotide's approval as covering MT-2

Fix: Different molecule, different structure, different evidence, different assessment. Nothing transfers.

✗ Accepting a sequence written without stereochemistry

Fix: L- and D-phenylalanine have identical mass. A peptide with the L form at position 4 is a different compound that mass spectrometry cannot distinguish.

✗ Assuming a non-selective agonist affects only its intended receptor

Fix: Five melanocortin receptors share binding surfaces across different tissues. Engagement elsewhere is the same pharmacology in another place, not a side reaction.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

What does MC1R control?: Which melanin type a melanocyte produces — eumelanin or pheomelanin — rather than simply how much pigment is made.
Why do MC1R variants cause red hair?: Loss-of-function variants leave the receptor signalling poorly, shifting production toward pheomelanin, which is red to yellow.
Does the receptor do anything else?: It has been associated with other melanocyte processes, which is what the 'and beyond' in the 2001 review's title refers to.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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