The KEGG object identifer or simply the KEGG identifier (kid) is a unique identifier for each KEGG object, which is also the database entry identifier. Generally it takes the form of a prefix followed a five-digit number as shown below:

<org> represents three- or four-letter organism code

Exceptions are GENES identifiers (including variant identifiers) and EC numbers for enzymes, which follow the general form of the the DBGET retrieval system (see About DBGET). Each entry in DBGET is identified by

<database>:<entry>

where <database> is the database name and <entry> is the entry name or the accession number (see DBGET for the list of database names and abbreviations).

The KEGG identifiers with database-dependent prefixes are often called D numbers, K numbers, C numbers, etc. Since these numbers are unique across the KEGG databases, <database> may be omitted in the DBGET system. For example,

D01441
dr:D01441
drug:D01441

are all equivalent.

In addition to the T number shown above, an organism in KEGG is given a three- or four-letter KEGG organism code, which is treated like a database name. Therefore, individual genes in an organism can be identified in the following way:

<org>:<gene>

where <org> is the KEGG organism code and <gene> is the KEGG GENES entry name. The KEGG organism code is also used as a prefix to identify organism-specific pathway maps or BRITE functional hierarchies (see: Pathway maps and Brite hierarchies).