The completion of the euchromatic portion of the human genome sequence was announced in Nature at the end of October. But even with this achievement behind us, we are reluctant to rest on our laurels. What should be sequenced next? For the human genome, the debate is divided between those who advocate the sequencing of genomes from more individuals — to learn more about genomic variation — and those who would like to see the human genome finally completed, by sequencing the heterochromatic regions.

The question of what to sequence next also applies to the choice of organism. Should we sequence closely or distantly related species? The platypus, for example, is scheduled to have its genome sequenced soon — this sequence and comparative genomics studies might give us important insights into mammalian evolution. If the platypus sex-determination mechanism is anything to go by (see Highlight on page 884), its genome might reveal findings that could be as bizarre as the animal's phenotype.

Among the organisms whose genomes have been sequenced is the dog — the subject of a comprehensive review by Nathan Sutter and Elaine Ostrander (page 900). Many diseases that dogs suffer from are confined to individual breeds, indicating that individual breeds are enriched for particular disease alleles. In fact, not only is the dog a model for many human diseases, perhaps most notably cancer and deafness, but the breeds can also serve as a model of geographically isolated human populations.

This issue is accompanied by a poster, by Roger Horton and colleagues, that depicts the definitive gene content of the human extended MHC. The poster, which was produced with support by Abbot Molecular Diagnostics, will be freely available until May 2005 (http://www.nature.com/nrg/journal/v5/n12/poster/MHCmap).