Chemical Abstracts Service (CAS)8, a division of the American Chemical Society, collects publicly disclosed information on chemical substances (primarily from scientific literature) and organizes them into a database called CAS Registry. The CAS Registry can be searched using CAS products such as SciFinder9 and STN10. In this database, unique identifiers are assigned to each chemical substance, providing an unambiguous way to identify a chemical substance when there are many possible systematic, generic, proprietary or trivial names. These unique identifiers are referred to as CAS Registry Numbers11,12, CAS RNs, or CAS Numbers.
CAS Numbers are numeric identifiers that can contain up to ten digits, divided by hyphens into three parts: the first consisting from two up to seven digits, the second consisting of two digits, and the third consisting of a single digit. These numbers themselves have no inherent chemical meaning (e.g., information on chemical structure), but are assigned in sequential order to new substances found by CAS scientists for inclusion in the database.
While CAS Registry Numbers have been widely used, the CAS Registry is a proprietary resource that comes with a non-trivial fee. These days many databases provide their contents free of charge to the general public, and unique identifiers used in these databases are getting popular. Examples are PubChem CIDs (Compound IDs), ChemSpider IDs, and ChEMBL IDs. Details about these databases will be discussed in Module 6.