- CAS registry number
CAS Registry Numbers are unique numerical identifiers assigned by the "Chemical Abstracts Service" to every chemical described in the open scientific literature (currently including those described from at least 1957 through the present) and including elements, isotopes, organic and inorganic compounds, organometallics, metals, alloys, coordination compounds, minerals, and salts; as well as standard mixtures, compounds, polymers; biological sequences including proteins & nucleic acids; nuclear particles, and nonstructurable materials (aka 'UVCB's- i.e., materials of Unknown, Variable Composition, or Biological origin). They are also referred to as CAS RNs, CAS Numbers, etc.
The Registry maintained by CAS is an authoritative collection of disclosed chemical substance information. Currently the CAS Registry identifies more than 56 million organic and inorganic substances and 62 million sequences, plus additional information about each substance; and the Registry is updated with an approximate 12,000 additional new substances daily. 
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 black-market names.
For example, Hoffman's "∂-Lysergsäure-Diethylamid", indexed by the CAS under the name "Ergoline-8-carboxamide, 9,10-didehydro-N,N-diethyl-6-methyl-, (8β)"-
and also known from the Merck Manual synthesis by names such as "9,10-Didehydro-N,N-diethyl-6-methylergoline-8β-carboxamide"- -
became most conveniently and discretely known as LSD-25 during initial pharmaceutical investigations.
But then during Sandoz' international promotion of the drug, it was subsequently labeled "Delysid" in the west. After 1963 Delysid attracted many additional, more colorful marketing names, initially confusing law enforcement and lay persons. Since 1989 LSD is again labeled for western research uses, currently available under another, historic trade name: "Lysergide".
On the other hand, CAS Numbers are not related to chemistry, are unrelated to any previous systems, and do not readily form phonetic analogs or synonyms. The 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.
A CAS 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 CAS Registry Number is separated by hyphens into three parts, the first consisting of up to 7 digits, the second consisting of two digits, and the third consisting of a single digit serving as a check digit. The check digit is found by taking the last digit times 1, the previous digit times 2, the previous 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: the checksum 5 is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) = 105; 105 mod 10 = 5.
Standardization of Complexity
• Isomers of a molecule are assigned discrete 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 enzymes known as alcohol dehydrogenases has 9031-72-5.
• A standard mixture of otherwise-identified compounds may receive a corporate CAS number; One example is mustard oil (8007-40-7).
The assigning agency, Chemical Abstracts Service (CAS) is a function of the American Chemical Society (ACS); CAS information is copyrighted by the ACS. Users wishing to incorporate CAS Numbers into databases should refer to CAS policy:
A User or Organization may include, without a license and without paying a fee, up to 10,000 CAS Registry Numbers or CASRNs in a catalog, web site, or other product for which there is no charge. The following attribution should be referenced or appear with the use of each CASRN: CAS Registry Number is a Registered Trademark of the American Chemical Society.
CAS number search engine
- CHEMINDEX Search via Canadian Centre for Occupational Health and Safety 
- ChemIDplus Advanced via United States National Library of Medicine 
- Common Chemistry TM  via Australian Inventory of Chemical Substances 
- European chemical Substances Information System  via the website of Royal Society of Chemistry 
- HSNO Chemical Classification Information Database via Environmental Risk Management Authority 
- Search Tool of Australian Inventory of Chemical Substances 
The lowest numeric CAS registry number is 50-00-0 corresponding to formaldehyde. Thus formaldehyde was potentially the first compound to receive a CAS registry number.
- International Union of Pure and Applied Chemistry
- Academic publishing
- Beilstein Registry Number
- Chemical database
- Chemical file format
- Dictionary of chemical formulas
- EC number (Enzyme Commission)
- EC# (EINECS and ELINCS)
- International Chemical Identifier (InChI)
- MDL number
- Registration authority
- UN number
- ^ CAS registry description, by the Chemical Abstracts Service
- ^ American Chemical Society. "CAS Registry and CASRNs". http://www.cas.org/expertise/cascontent/registry/regsys.html#q2. Retrieved 25 July 2009.
- ^ CAS Registry Number and Substance Counts
- ^ American Chemical Society (CAS) (2009-06-18). "CAS Information Use Policies (effective July 2, 2009)". http://www.cas.org/legal/infopolicy.html#authorized. Retrieved 2010-02-10.
- ^ Canadian Centre for Occupational Health and Safety. "CHEMINDEX Search". http://ccinfoweb.ccohs.ca/chemindex/search.html. Retrieved 13 July 2009.
- ^ United States National Library of Medicine. "Advanced". http://chem.sis.nlm.nih.gov/chemidplus/. Retrieved 1 December 2009.
- ^ American Chemical Society. "Substance Search". http://www.commonchemistry.org/. Retrieved 8 July 2009.
- ^ National Industrial Chemicals Notification and Assessment Scheme. "AICS Detailed Help / Guidance Notes". http://www.nicnas.gov.au/Industry/AICS/Search/AICS_Detailed_Help.asp#CAS_Number_Search. Retrieved 8 July 2009.
- ^ European Commission Joint research Centre. "ESIS : European chemical Substances Information System". http://ecb.jrc.ec.europa.eu/esis/. Retrieved 11 July 2009.
- ^ Library & Information Centre. "Finding a CAS Registry Number". http://www.rsc.org/Library/Features/Tips/CASNumbers.asp. Retrieved 11 July 2009.
- ^ Environmental Risk Management Authority. "HSNO Chemical Classification Information Database". http://www.ermanz.govt.nz/Chemicals/ChemicalSearch.aspx. Retrieved 14 July 2009.
- ^ National Industrial Chemicals Notification and Assessment Scheme. "AICS Search Tool". http://www.nicnas.gov.au/Industry/AICS/Search.asp. Retrieved 11 July 2009.
- CAS registry description, by the Chemical Abstracts Service
To find the CAS number of a compound given its name, formula or structure, the following free resources can be used:
- Chemical Synthesis Database
- NLM,NIH ChemIDplus
- NIST Chemistry WebBook
- R&D Chemicals
- NCI Database Browser
- NCI/CADD Chemical Identifier Resolver
- ChemSub Online (Multilingual chemical names)
- European chemical Substances Information System (ESIS) - useful for finding EC#
- NIOSH Pocket Guide to Chemical Hazards, index of CAS numbers
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