#4 InChIKey prefix

[Marcus Ennis]

Is the prefix "InChIKey=" considered to be part of the InChIKey, just as the "InChI=" is an integral part of the InChI? Some databases include the prefix (e.g. ChEBI), whereas others omit it (e.g. NIST Chemistry WebBook). Also, ACD/Name omits it when generating an InChIKey for a structure. The waters are further muddied by the inclusion in the InChI User's Guide (Overview) of a statement that 'The standard InChI is designated by the prefix: “InChI=1S/……….. “ ' but that merely 'The InChIKey consists of two blocks..........', whereas the examples given show inclusion of prefixes for both InChI and InChIKey. Could I have please some guidance on this?