Chemical Entities of Biological Interest (ChEBI) is a freely available dictionary of 'small molecular entities'. The term 'molecular entity' encompasses any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity. The molecular entities in question are either products of nature or synthetic products used to intervene in the processes of living organisms (either deliberately, as for drugs, or unintentionally', as for chemicals in the environment). The qualifier 'small' implies the exclusion of entities directly encoded by the genome, and thus as a rule nucleic acids, proteins and peptides derived from proteins by cleavage are not included.