CAS number
In short A unique registry number assigned to a chemical substance by the Chemical Abstracts Service.
The Chemical Abstracts Service, a division of the American Chemical Society, gives every distinct substance it records a registry number in the form 137525-51-0. The number identifies the compound regardless of trade name, spelling or language, so two product pages showing the same CAS number describe the same molecule.
The number itself carries no meaning about structure or size: it is assigned in sequence. The final digit is a check digit calculated from the others, which means a mistyped or invented number usually fails a simple lookup.
Research blends of several peptides have no CAS number of their own, because a CAS number designates exactly one substance. On those pages we list the components instead. Laboratory reagents such as bacteriostatic water are mixtures as well and are described by composition.
On every UKPeps product page the CAS number appears in the specification table and in the page's structured data, so it can be checked against public chemical databases before ordering.