Enumerations for _Chem_comp_identifier.Type (An open enumeration) 
CAS REGISTRY NUMBER 'Chemical Abstracts Registry Number' 
COMMON NAME 'Common chemical name' 
MDL Identifier 'Molecular Design Limited Identifier' 
PUBCHEM CID 'PubChem compound identifier' 
PUBCHEM Identifier 'PubChem accession number' 
PUBCHEM SID 'PubChem substance identifier' 
SYSTEMATIC NAME 'Systematic chemical name'