Annotation of output identifiers. An example demonstrating annotation of output PubChem Compound identifiers (b-e), when the KEGG Compound identifier for D-glucose (a) is input to a mapping application. The preferred output identifier is for D-glucose (b), but an identifier for alpha-D-glucose (c) is also valid since it is a D-glucose. An identifier for a generic hexose (d), however, is not valid. Finally, an identifier for phospholactic acid (e), which is a completely different compound, is incorrect.