The rat has played a two-sided role in the history of human health. Infamous in the wild as a carrier of deadly illnesses such as the bubonic plague, the rat has also made its name in the laboratory as an indispensable model for studying human biology and disease and for developing and testing new drugs. Now it has become the third mammalian species (after humans and mice) to have its genome sequenced, promising greater insight into biomedicine, comparative biology and evolution.

The consortium of researchers involved in the Rat Genome Sequencing Project, led by Richard A. Gibbs of the Baylor College of Medicine, produced a 90-percent-complete "draft" sequence of the genome of the Brown Norway strain of the lab rat Rattus norvegicus. According to their report, published today in the journal Nature, the size of the rat genome falls between that of the mouse and the human, but all three contain a similar number of genes, 90 percent of which are shared among them.

Scientists have long recognized that most human genes that are associated with diseases have parallels in the rat genome, but the exact locations of these genes in the rat were in many cases unknown. "The rat genome data [have] improved the utility of the rat model enormously," the team writes. "Now that near-complete knowledge of the rat gene content is realizable, individual researchers have a data source for the rat 'parts list' that can be explored with the high degree of confidence and precision that is appropriate for biomedical research." The results also suggest that rodent genes may mutate faster than human ones, perhaps hinting at diverse evolutionary mechanisms among mammals. "Any one organism is a unique solution to the evolutionary challenges it has faced," Gibbs says, and comparing them affords a look through the keyhole at the history of each species.

Continuing its rodent work, the consortium is analyzing differences among various rat strains to determine the functions of specific genes. Meanwhile, efforts to sequence the genomes of other mammals--including the chimpanzee, macaque, dog, opossum and cow--are under way. --Alla Katsnelson