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We now know what it means to be a mouse. An international team of publicly funded researchers has decoded the mouse genome, give or take a few loose ends. Analyses so far show that the mouse’s 20 chromosomes contain some 30,000 genes, about the same as the human genome, and that people and mice share three-quarters of their genes. So the mouse sequence should provide a key to understanding how human genes work. The sequencing, by collaborators at the Sanger Centre in Cambridge and at institutes in the US, is 96 per cent complete. The US company Celera has already mapped the mouse genome, but scientists who want to use its data have to pay.