A CAS Registry Number (CAS RN) is a unique identification number assigned by the Chemical Abstracts Service (CAS) in the US to every chemical substance described in the open scientific literature since 1957, plus some additional substances going back to the early 1900s. It is a numeric identifier that can contain up to 10 digits, divided by hyphens into three parts. The right-most digit is a check digit used to verify the validity and uniqueness of the entire number. CAS numbers are simple and regular, convenient for database searches, and 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. They are used to identify chemical substances in many databases, both public and private, as well as other chemical inventory listings. CAS numbers are an important part of any Safety Data Sheets (SDS) and are required information on SDSs in the 16-part standardized GHS format that includes CAS numbers as a required element in Section 3 (Composition/information on ingredients) of the SDS.