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CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences, mixtures and alloys. They are also referred to as CAS numbers or CAS RNs. Chemical Abstracts Service (CAS), a division of the American Chemical Society, assigns these identifiers to every chemical that has been described in the literature. CAS also maintains and sells a database of these chemicals, known as the CAS registry. About 23 million compounds have received a CAS number so far, with about 4,000 new ones being added each day. The intention is to make database searches more convenient, as chemicals often have many names. Almost all molecule databases today allow searching by CAS number.
A CAS registry number is separated by hyphens into three parts, the first consisting of up to 6 digits, the second consisting of two digits, and the third consisting of a single digit serving as a checksum. The numbers are assigned in increasing order and do not have any inherent meaning. The checksum is calculated by taking the last digit times 1, the next digit times 2, the next digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is 7732-18-5, and the checksum is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) mod 10 = 105 mod 10 = 5.
Different isomers of a molecule receive different CAS numbers: D-glucose has 50-99-7, L-glucose has 921-60-8, α-D-glucose has 26655-34-5, etc. Occasionally, whole classes of molecules receive a single CAS number: the group of alcohol dehydrogenases has 9031-72-5. An example of a mixture with a CAS number is mustard oil (8007-40-7).
When using CAS numbers for database searches, it is useful to include the numbers of closely related compounds. For instance, to search for information about cocaine (CAS 50-36-2), one should include cocaine hydrochloride (CAS 53-21-4), since this is the most common form of cocaine when used as a drug.
To find the CAS number of a compound given its name, formula or structure, the following free resources can be used: