Orthology is the cornerstone of comparative genomics and gene function prediction. OrthoDB aims to classify protein-coding genes from the increasing number of available sequenced genomes into groups of orthologs descended from a single gene of the last common ancestor (LCA) of each clade of species. Applying this concept to the hierarchy of LCAs along the species phylogeny results in multiple levels of orthology: the more closely-related the species, the more finely-resolved the orthologous relations.