A CAS Registry Number, also referred to as CASRN or CAS Number, is a unique numerical identifier assigned by the Chemical Abstracts Service to every chemical substance described in the open scientific literature, including organic and inorganic compounds, minerals, isotopes, alloys and nonstructurable materials. CASRNs are generally serial numbers, so they do not contain any information about the structures themselves the way SMILES and InChI strings do. The registry maintained by CAS is an authoritative collection of disclosed chemical substance information. It currently identifies more than 158 million unique organic and inorganic substances and 67 million protein and DNA sequences, plus additional information about each substance. It is updated with around 15,000 additional new substances daily.
Use
Historically, chemicals have been identified by a wide variety of synonyms. Frequently these are arcane and constructed according to regional naming conventions relating to chemical formulae, structures or origins. Well-known chemicals may additionally be known via multiple generic, historical, commercial, and/or -market names. CAS Registry Numbers are simple and regular, convenient for database searches. They offer a reliable, common and international link to every specific substance across the various nomenclatures and disciplines used by branches of science, industry, and regulatory bodies. Almost all molecule databases today allow searching by CAS Registry Number.
Format
A CAS Registry Number has no inherent meaning but is assigned in sequential, increasing order when the substance is identified by CAS scientists for inclusion in the CAS REGISTRY database. A CASRN is separated 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 serving as a check digit. This current format gives CAS a maximum capacity of 1,000,000,000 unique identifiers. The check digit is found by taking the last digit times 1, the preceding digit times 2, the preceding digit times 3 etc., adding all these up and computing the summodulo 10. For example, the CAS number of water is 7732-18-5: the checksum 5 is calculated as = 105; 105 mod 10 = 5.
Granularity
Stereoisomers and racemic mixtures are assigned discrete CAS Registry Numbers: L-epinephrine has 51-43-4, D-epinephrine has 150-05-0, and racemic DL-epinephrine has 329-65-7
Different phases do not receive different CASRNs, but different crystal structures do
Commonly encountered mixtures of known or unknown composition may receive a CASRN; examples are Leishman stain and mustard oil.
Some metals are discerned by their oxidation state, e.g. the element chromium has 7440-47-3, the trivalent Cr has 16065-83-1 and the hexavalent Cr ion has 18540-29-9.
Occasionally whole classes of molecules receive a single CASRN: the class of enzymes known as alcohol dehydrogenases has 9031-72-5.
