High Quality Content by WIKIPEDIA articles! 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.