A CAS Registry Number identifies one specific chemical substance in the CAS REGISTRY, the largest curated substance database. It is not a formula and carries no chemistry; it is a label assigned by CAS. Verify a peptide's number by confirming the check digit, then resolving it at CAS or PubChem and reading the structure back against the product record.
- A CAS Registry Number identifies one substance in the CAS REGISTRY; it encodes no chemistry and is assigned by CAS, not by the manufacturer.
- The last digit is a check digit: weight each preceding digit by its position from the right, sum, and take the remainder mod 10.
- Salts, hydrates, metal complexes and stereoisomers are distinct substances and carry distinct registry numbers where registered.
- Most research peptides ship as acetate or trifluoroacetate salts while records print the free-peptide number; the counterion should be stated separately.
- A number that resolves proves the label is precise, not that the vial contains the substance; only the lot's mass spectrometry shows that.
What a CAS number is, and what it is not
A CAS Registry Number is an identifier assigned by CAS, a division of the American Chemical Society, to one substance in the CAS REGISTRY. The registry has been maintained since 1965 and holds well over two hundred million organic and inorganic substances. Each number is unique to one substance and one substance only, and the assignment is made by CAS's own scientists reviewing the structure, not by the manufacturer of the material. That is the whole value of the thing. A name can be spelled six ways and a formula can describe several isomers, but a registry number points at exactly one entry.
It is worth being clear about what the number does not carry. There is no chemistry encoded in it. The digits are assigned sequentially as substances enter the registry, so a higher number means only that the substance was registered later, and two adjacent numbers can belong to a peptide and a pigment. The number does not state a formula, a molecular weight, a sequence, or a purity. It is a key into a database, and the database holds the chemistry.
For a peptide buyer the practical consequence is that a CAS number on a product record is a claim about identity that can be checked, which puts it in a different class from a marketing name. A supplier who writes 137525-51-0 next to BPC-157 is asserting that the material is the pentadecapeptide CAS registered under that number. Whether the vial contains it is a separate question for the analytical documentation, but at least the claim is precise enough to be wrong.
Reading the format and the check digit
A registry number has up to ten digits in three groups separated by hyphens. The first group has two to seven digits, the second always has two, and the third is a single check digit. Water is 7732-18-5; tirzepatide is 2023788-19-2. The hyphens are part of the identifier and a record that drops them, or writes the groups in the wrong lengths, is either a transcription error or something that was never a registry number.
The check digit is computable, which makes it the first test to run. Take every digit except the last, number them from the right starting at one, multiply each digit by its position, sum the products, and take the remainder on division by ten. That remainder must equal the check digit. For water the six digits 7, 7, 3, 2, 1, 8 give 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 105, and 105 modulo 10 is 5, which matches.
| Substance | Registry number | Weighted sum | Sum mod 10 | Check digit |
|---|---|---|---|---|
| BPC-157 | 137525-51-0 | 130 | 0 | 0 |
| Semaglutide | 910463-68-2 | 152 | 2 | 2 |
| Tirzepatide | 2023788-19-2 | 152 | 2 | 2 |
| TB-500 | 885340-08-9 | 189 | 9 | 9 |
A number that fails the check digit is not a registry number, full stop, and the record it appears on was assembled by someone who did not look it up. A number that passes has cleared a low bar. Passing proves the digits are internally consistent; it says nothing about whether they belong to the substance beside them. The next two steps are the ones that matter.
Resolving the number at the source
CAS publishes a free lookup on its own site that returns the substance name and structure for a registry number, and that is the authoritative answer. PubChem, maintained by the National Library of Medicine, indexes CAS numbers as synonyms on its compound records and is the more useful working tool, because it shows the structure, the molecular formula, the computed mass, and the depositor names on one page. Searching PubChem for BPC-157 returns a compound record listing the registry number among its identifiers, the fifteen-residue sequence, and a formula the product record can be read against.
The reading is the point. A number that resolves is only useful if what it resolves to is what the vial claims to be. Compare four things between the resolved record and the product record: the sequence or structure, the molecular formula, the presence or absence of a counterion or complexed metal, and the name the registry actually uses. The last catches the most errors. GHK-Cu resolves to the copper complex of the tripeptide glycyl-L-histidyl-L-lysine; a record that carries the GHK-Cu number but describes the copper-free tripeptide is describing a different substance with a different registry number.
Two things a lookup will not do. It will not tell you the material is real, and it will not tell you the material is pure. A registry number is an identity claim about what the substance is meant to be; the certificate of analysis, and specifically the mass spectrometry on it, is the evidence that a particular lot is that substance. The Verify a COA page covers how to confirm the certificate at the laboratory that issued it. This page only covers whether the label on the claim is the right label.
Why salts, forms and complexes have their own numbers
The registry is organised by substance, and a substance is defined by its composition and structure, not by the parent molecule people talk about. That means a free peptide, its acetate salt, its trifluoroacetate salt, and a hydrate of any of them are different substances and, where they have been registered, carry different numbers. The same is true of a metal complex against the free ligand, and of a stereoisomer against its enantiomer. CAS states this directly in its description of the registry: the number applies to the specific substance as described, and a change in composition is a change in substance.
Research peptides are almost always supplied as a salt, because that is what comes off the purification column, and the salt is usually acetate or trifluoroacetate depending on the final buffer. A great many product records carry the registry number of the free peptide while the vial holds the acetate. That is not fraud, it is the convention across the whole research-chemical trade and across a good deal of the pharmaceutical literature, but it is an imprecision a buyer should know about.
Where a salt-specific number exists and the supplier knows the counterion, the salt-specific number is the more honest one to print. Where the supplier prints the parent number, the counterion should appear separately on the record, because it changes the net peptide content and therefore the arithmetic of every solution made from the vial.
The purity testing guide explains why the counterion is invisible to the HPLC purity figure and why net content is reported separately. The registry-number question and the counterion question are the same question seen from two sides: what, exactly, is the substance in this vial.
Names, sequences and the identifier hierarchy
A peptide record will usually carry several ways of naming the same thing, and they are not equally trustworthy. In roughly descending order of precision: the amino-acid sequence written in IUPAC-IUBMB notation with its termini and modifications stated; the CAS Registry Number; the molecular formula; a systematic or semi-systematic name; a common name; and a trade or catalog name. The sequence is the most informative because it is the structure. The registry number is the most checkable because it is one number in one database. The formula is useful but not unique, since isomers share it. Names drift.
The IUPAC-IUBMB recommendations on amino acid and peptide nomenclature set the rules for the sequence line: three-letter or one-letter residue codes, left to right from the N-terminus, terminal modifications written outside the chain, and a stated configuration where it is not L. A record whose sequence line follows those rules can be compared residue by residue against the resolved registry entry. A record whose sequence line is missing, or is a picture rather than text, cannot be, and the registry number is then doing all the work on its own.
Common names are where most confusion lives. TB-500 is a trade name for a fragment of thymosin beta-4, not for the full protein, and the two have different registry numbers. CJC-1295 with and without DAC are two substances. Melanotan I and Melanotan II are different molecules with a shared prefix. A registry number on the record settles which one is meant, which is why a record without one should be treated as incomplete rather than merely informal.
A verification routine that takes four minutes
The routine below is what a laboratory buyer can do before ordering, with nothing but a browser, and it catches the large majority of identity errors on research peptide records. None of it requires trusting the supplier.
- Check the format. Three hyphen-separated groups; the middle group has two digits; the last is one digit. Anything else is not a registry number.
- Run the check digit. Weighted sum of the leading digits, remainder mod 10, compare. A failure ends the exercise.
- Resolve it. Search the number at CAS or on PubChem and confirm the substance that comes back is the substance named on the record, including the salt or complex form.
- Read the structure back. Compare sequence, formula and counterion between the resolved entry and the product record. Note any mismatch in writing.
- Carry it to the certificate. The registry number tells you what the material is supposed to be. Only the lot's analytical report tells you whether it is. Read the mass result against the formula the number resolved to.
If a record fails any step, ask the supplier which substance they meant and why the record says otherwise. A supplier that can answer in a sentence has a record-keeping problem, which is fixable. A supplier that cannot has an identity problem, which is not yours to fix. Either way the question was worth asking before money moved, and a supplier that objects to being asked has told you something too.
Fragments, parents, isomers and blends
Four situations on research peptide records deserve their own numbers and rarely get them. The first is a fragment sold under a name that belongs to a larger molecule. TB-500 is a seven-residue fragment of thymosin beta-4; PubChem lists 885340-08-9 for the fragment, and the full forty-three-residue protein is a different substance with a different entry. A record that carries the protein's identifiers for the fragment, or the fragment's for the protein, has named the wrong substance. The second is stereochemistry. A sequence built from D-amino acids is a different substance from its L-form and registers separately, and a record for a D-isomer that prints the L-form's number has made the same mistake in a different direction.
The third is the blend. A product that combines two peptides in one vial, such as a BPC-157 and TB-500 blend, is a mixture, and mixtures are not substances: there is no single registry number for the blend and there never will be. The honest record carries the number of each component and states the proportion, which is also what the composition section of the safety data sheet needs. A single number printed against a blend is either the number of one component or an invention.
The fourth is the metal complex. GHK-Cu is the copper complex of the tripeptide glycyl-L-histidyl-L-lysine, and the complex and the free tripeptide are two substances. PubChem's record for the complex, at the time of writing, lists no registry number among its synonyms at all. That is worth knowing before trusting a number printed for it on a product record: the easiest verification route returns nothing, so the number has to be resolved at CAS directly, and a supplier that printed it should be able to say where it came from.
How the registry number appears on our records
Each product record in the LabFirst catalog states the substance it lists by name and, where the entity is verified, by sequence and registry number, with the counterion stated separately when it is known. Where a field has not been verified against a primary source it is left blank rather than filled from a supplier's sheet, because a blank is honest and a copied number is a claim we did not check. The compliance page sets out the record standard as a whole; the lot-level analytical documentation is described on the Verify a COA page, which is where the identity claim a registry number makes gets tested against a real vial.
Nothing on this page, or on any product record, describes a use. A registry number identifies a chemical substance and nothing more. FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. The catalog supplies research materials to laboratories; identity documentation exists so a laboratory can confirm what it received, and for no other purpose.
What is Melanotan II studied for?
Published research on Melanotan II investigates the areas below — which is a different question from what Melanotan II will do for anyone, a claim about a living system that nothing on this site is sold for.
What it is. A man-made ring-shaped peptide seven building blocks long that switches on melanocortin receptors indiscriminately.
What the research looks at. It appears in melanocortin receptor research about binding and selectivity.
How it is thought to work. A ring-shaped relative of alpha-MSH that acts across several melanocortin receptor types rather than picking one. That lack of selectivity is its defining feature and the reason it is studied as a broad melanocortin activator.
What is not established. No approved drug product. Regulators in several countries have issued public warnings about unapproved products sold under this name. This record covers how it behaves at its receptors and does not describe use in a person.
The full record, including the certificate for the lot in stock, is on the Melanotan II product page.
Common questions
Does a CAS number tell me the purity of a peptide?
Why does the acetate salt have a different CAS number from the free peptide?
What if the CAS number on a product record fails the check digit?
Where can I look up a peptide CAS number for free?
Is a peptide without a CAS number a fake?
Sources
- CAS, CAS REGISTRY: what is a CAS Registry Number. The registry's own description of what a registry number identifies, how substances are distinguished, and the scope of the database.
- PubChem compound record: BPC-157. Example of a resolved record showing the registry number among the identifiers alongside the structure, formula and computed mass.
- PubChem compound record: tirzepatide. Second worked example used for the check-digit table and for reading a formula back against a product record.
- IUPAC-IUBMB Joint Commission on Biochemical Nomenclature, Nomenclature and Symbolism for Amino Acids and Peptides. The rules for writing a peptide sequence line, which is the identifier a registry number is read against when a record carries both.
- FDA / ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. The framework separating identity from purity and quantity in a specification, which is why a registry number and a certificate answer different questions.