CAS Registry Number

A CAS Registry Number is the index number the Chemical Abstracts Service assigns to each substance it records. Its up to ten digits encode nothing about the molecule; numbers are issued in sequence. The last digit is a checksum: number the preceding digits from the right starting at one, multiply each by its position, and the last digit of the sum is the check digit. For semaglutide, 910463-68-2, that is 8 + 12 + 9 + 24 + 20 + 0 + 7 + 72 = 152, ending in 2.

The unit of registration is the substance, not the drawn molecule. A peptide, its acetate salt and its trifluoroacetate salt each get their own number, as do stereoisomers; octreotide and octreotide acetate are different entries. Registration ignores status: an approved drug, a failed phase 3 candidate and a grey-market peptide share the format, and some research peptides have none.

The number is an unambiguous cross-reference: from it you can pull the structure, formula and molecular weight it stands for and check them against a label or certificate, and tell whether a document means the free peptide or a salt, which shifts molecular weight and net peptide content. It says nothing about quality, purity, approval or safety; it is a call number, not a pedigree.

That is why a wrong number on a certificate of analysis matters. A genuine certificate is generated from a specification, so its number is copied, not typed. The checksum catches every adjacent-digit transposition and most single-digit slips, so a failing number was keyed by hand; one that passes but names the acetate salt beside the free peptide's molecular weight, or another peptide entirely, came from someone else's template. Both point to fabrication: the number certifies the description, never the vial.