The virus (T40000) category of the KEGG GENES database is generated from the RefSeq database and given KO annotation. RefSeq GeneID's are used as gene identifiers and the organism code "vg" is used for the entire set of viral genes. For example, vg:3783681 is BALF5 (DNA polymerase catalytic subunit) gene of human herpesvirus 4 (Epstein-Barr virus) with KO annotation of K18964.

Some KOs in KEGG organisms are shared by viruses, but there are many unique KOs that are found only in viruses. Based on experimental evidence, virus-specific KOs are defined especially for those viral proteins that are relevant in human diseases.

• Viral proteins

All the viruses present in KEGG GENES are classified according to the NCBI taxonomy identifiers as shown below.

• KEGG viruses in the NCBI taxonomy

Selected viruses are then given T4 identifiers and are now part of the KEGG GENOME database. Note that each T4 entry is a collection of virus genomes with the same taxonomy ID. Note also that T40000 is not part of KEGG GENOME; it simply represents the virus category of KEGG GENES.

• KEGG selected viruses

The selected viruses have relevance to human diseases or plant diseases. T4 entries for human-disease causing viruses are linked from KEGG DISEASE and KEGG BRITE, while those for plant-disease causing viruses appear in KEGG BRITE.