What Is a CAS Number, and Why Can One Peptide Have Two?

Who assigns CAS numbers, and what do they identify?

CAS, a division of the American Chemical Society, maintains the CAS Registry. The registry began in 1965 and organizes substances reported in scientific literature and other chemical information sources. CAS explains that the number itself has no chemical meaning. It is an identifier, not a formula, structure, or measurement of purity.

That distinction matters because a single compound can be described by a systematic name, a common name, or a research code. A registry identifier helps connect those names to a specific registered substance. CAS has described its database as containing more than 290 million substances, although published counts can differ between CAS pages and change over time. The exact total is less useful to a reader than the distinction between the complete registry and smaller free search collections.

In organic and natural-product chemistry, I regularly encounter multiple names for related structures when reviewing literature and analytical records. A registry number can help organize a search, but I would not use the number alone to establish what is actually in a sample.

How is a CAS number written and checked?

A CAS number has three groups separated by hyphens. The first contains two to seven digits, the second contains two, and the final digit is a check digit. This last digit is calculated from the other digits. It helps detect transcription mistakes, but it does not establish chemical identity.

Take the number 137525-51-0, associated with BPC-157. Read the digits before the last hyphen from right to left, multiplying them by 1, 2, 3, and so on. The calculation is 1×1 + 5×2 + 5×3 + 2×4 + 5×5 + 7×6 + 3×7 + 1×8 = 130. The final digit of 130 is 0, matching the check digit.

Now consider a typing error: 137525-15-0. The weighted sum becomes 126, so its check digit should be 6 rather than 0. The written number fails the test. This method catches many common errors, but a different, correctly formed number can still refer to the wrong substance.

The number associated with BPC-157

137525-51-0

• 1×88
• 3×721
• 7×642
• 5×525
• 2×48
• 5×315
• -
• 5×210
• 1×11
• -
• 0check

8 + 21 + 42 + 25 + 8 + 15 + 10 + 1= 130

The last digit of 130 is 0, matching the check digit 0.

The same number with a typing error

137525-15-0

• 1×88
• 3×721
• 7×642
• 5×525
• 2×48
• 5×315
• -
• 1×22
• 5×15
• -
• 0check

8 + 21 + 42 + 25 + 8 + 15 + 2 + 5= 126

The last digit of 126 is 6, so the check digit should be 6, not 0.

BPC-157 on this siteBPC-157 ReferenceBPC-157 ResearchRegeneration Peptides

Why can one peptide have two CAS numbers?

CAS registers distinct chemical substances, including many salts separately from their parent compounds. A salt contains charged components associated with one another. For a peptide with positively charged groups, acetate can be a negatively charged counter-ion. The amino acid sequence can remain the same while the composition of the registered substance changes.

BPC-157 offers a useful example. PubChem has separate compound records for the peptide and for a specifically represented 1:1 acetate form:

• BPC-157

  CAS number

  137525-51-0

  PubChem CID9941957

  FormulaC₆₂H₉₈N₁₆O₂₂

• BPC-157 acetate (1:1 record)

  CAS number

  216441-37-1

  PubChem CID155977614

  FormulaC₆₄H₁₀₂N₁₆O₂₄

The formulas differ by C2H4O2, the composition of acetic acid. That difference is reflected in molecular weight, as explained in why the salt form changes the molecular weight. PubChem’s 1:1 entry is a database representation; it should not be assumed to describe the exact counter-ion ratio of every laboratory preparation.

CAS registration criteria also distinguish certain stereochemical forms and specific-ratio from indefinite-ratio salts. Physical state alone does not necessarily create a new registry entry. The lesson is that similar names do not guarantee identical registered substances.

What does a CAS number tell you on a certificate or specification?

A certificate of analysis or technical specification may include a CAS number alongside a name, molecular formula, and other identifying information. The number is useful only when it matches the form the document describes. A certificate naming an acetate salt should not automatically be interpreted as describing the free peptide merely because both share a familiar peptide name.

When reviewing chemical documentation, I prefer to compare the name, formula, and analytical description together. This is a general practice from my experience with organic synthesis, chromatography, and characterization. I am not claiming to have personally resolved a BPC-157 registry discrepancy. The important point is that a valid identifier and a chemically matching identifier are different things.

For readers comparing documents, how to read a certificate of analysis explains how identification fields fit with other reported results.

Where can you check a CAS number?

CAS Common Chemistry is a free search resource operated by CAS, but it covers only a subset of the full registry. A missing result there does not prove that a substance lacks a CAS number. The full CAS Registry is broader and generally accessed through CAS information products.

PubChem is another useful public database. Its records include molecular formulas, structures, names, and identifiers supplied from different sources. Searching a CAS number may lead to a relevant compound record, but PubChem is not the CAS Registry itself. Synonyms and identifiers from depositors can vary, so a result should be checked against the chemical form and formula.

For example, the BPC-157 acetate record contains a curated association with 216441-37-1, also reflected in the FDA substance registration record. It also contains another CAS-formatted synonym from depositors. Without an authoritative CAS Registry check, that second identifier should not be assigned a specific chemical explanation. This is a useful reminder that a database search is the beginning of verification, not the end.