One of the main objectives of the KEGG project has been to uncover higher level systemic functions of the cell and the organism from genomic and molecular-level information. The basis for genome annotation in KEGG, which is continuously performed for all sequenced genomes, is the KO system consisting of manually defined ortholog groups that correspond to individual nodes in the KEGG pathway maps and the BRITE functional hierarchies. Once genes are assigned KO identifiers or K numbers by the ortholog annotation procedure described below, the collective body of K numbers can be mapped to KEGG pathway maps and BRITE functional hierarchies, highlighting any subsystems present and enabling higher level functional interpretation of the genome.
