SHANGHAI--Geneticist Xiaohui Wu looks through a window into a clean room on the campus of Fudan University here and proudly points to a growing collection of mutant mice. To a visitor, the 4000 cages and 20,000 mice representing 400 mutant strains look pretty impressive. To Wu, the scale of the operation is a frustrating limitation.\n"We plan to mutate 70% of the mouse genome over the next 5 years," he says. Yet, their current facilities are filled to capacity. A new building will provide space for 10,000 more cages. But Wu needs 50,000 more, enough for about 100,000 mutant mice. Those cages, he says, require a lot more space and "a lot of money."\nThroughout the world, researchers are setting up programs to shut down the mouse genome gene by gene to learn what each gene does (see main text). The Fudan University mouse facility--a joint effort with Yale University--is shooting to be a key player and hopes to team up with the U.S. National Institutes of Health (NIH) Knockout Mouse Project.\nThe driving force behind the tentatively named Mammalian Functional Genome Project is Tian Xu, a geneticist at Yale University School of Medicine who is also an adjunct professor at Fudan. The Fudan-Yale group, along with colleagues at the University of Colorado, Boulder, and Duke University in Durham, North Carolina, has come up with an efficient way to knock out mouse genes. They use a transposon, a short segment of DNA that invades genomes, sometimes inserting itself into a gene and deactivating it.\nDevelopmental biologists have used transposons to disable genes in plants, worms, and fruit flies for years, but they had not found one that worked well in mammals. After 8 years of searching, Xu and his colleagues found "piggyBac," which was first identified in the cabbage looper moth by molecular virologist Malcolm Fraser of the University of Notre Dame in Indiana. "We don't know why it works," says Xu. But it does. The group reported its finding in the 12 August 2005 issue of Cell.\nThe technique is similar to gene trapping in that it randomly disables genes. But using a transposon avoids the laborious manipulation of embryonic stem cells required by other knockout techniques. The researchers made a line of mice that carry both the transposon and DNA that causes the transposon to move. When they mate these mice with wildtype mice, the transposon hops to a new place, preferably to a gene. "All you need to do is just breed mice, and each has different genes mutated," Xu says. This approach can hit genes other knockout approaches tend to miss, he adds.\nAlso, the Fudan-Yale group has put the gene for red fluorescent protein into the transposon. Mice that wind up with the transposon in their genomes are pink under ultraviolet light. "You just look at it, and you can tell" if the genome is carrying the transposon, Xu says.\nThe Fudan-Yale team opted to set up its large-scale mouse facility in China to save money. Xu estimates that this project could cost one-fifth to one-fourth what it would cost in the United States. But it is still not cheap, and international researchers are impressed by the $12.5 million already pledged from national and local government funding agencies. "I think it's great that [the Chinese] are doing this," says Phil Soriano, a developmental biologist at the Fred Hutchinson Cancer Research Center in Seattle, Washington. Wolfgang Wurst, director of the Institute of Developmental Genetics at the National Research Center for Environment and Health in Munich, Germany, thinks the project is a welcome indication of China's increasingly international orientation. "It is a sign that they are serious research partners," he says.\nTo leverage support from China itself, Xu and Wu are asking for $30 million from NIH to start mass-producing, preserving, and distributing mutant mice. For the cost of shipping and handling, researchers will receive frozen embryos or sperm, with no intellectual-property-rights restrictions attached. Also, the NIH money would go a long way toward producing the 100,000 strains of transposon-modified mice. Wu and Xu need that number of strains to be sure they have 20,000 genes covered, because the transposon also lands on noncoding regions. If they don't get NIH funding, they may have to recoup some costs by charging fees or placing restrictions on mutant mice, Wu says.\nAt this point, the other programs are simply making knockout strains, But here, researchers are busy screening the more than 400 mutant mice they have generated over the past year, looking for phenotypes from neurophysiological, immunological, and disease angles, among others. That information will go up on the Web prior to publication, making it easier for potential users to see which mouse will best suit their needs, the duo point out. Four hundred mutants is about the limit until the team's new facility comes on line. After that, the view through these new clean-room windows will get even more interesting.