Provides orthologs, genes inherited by extant species from a single gene in their last common ancestor. OrthoDB includes a total of 5756 species, providing ortholog groups for the clades of: 3663 bacteria, 330 metazoans, 227 fungi, 345 archaea and 1157 viruses. Among the metazoans, there are 172 vertebrates and 133 arthropods. There are 290 orthology levels, that were retrieved from the NCBI Taxonomy. The web resource presenting the OrthoDB data enables identified user sessions to analyze custom data sets in the context of the available orthology data, as well as to generate publication quality comparative genomics reports.