So we now have the complete (well almost) sequence of the human genome, to add to those of a fruit fly, a worm, a plant, yeast, and countless bacteria. What now? No doubt there will be more complete sequences. The mouse genome should be along shortly and zebrafish should be soon after that. But what will all this sequence tell us and how can we use it to get what we want? At one level we can use sequence as we have always used sequence, to work out where genes are in the genome, where and when they are expressed, whether they might be involved in disease phenotypes, etc. Comparing sequences between different organisms, we can work out which bits of genes are more likely to be functional, which non-coding regions are likely to be functional, etc. We can also work out how organisms are related to one another. Complete genomes allow us to do the same with a larger sample size and in some cases more easily. The thesis ofComparative Genomics, however, is that we can also use the new genomic sequence to look at a new dimension of problems and to look at old problems in a new way.

Consider, for example, the old problem of …