Wanted for long-term occupancy: clean, secure home, must have ample freezer space, 20,000 bedrooms, starting July 2007. An ambitious plan getting under way to learn the function of every gene in the classic lab mouse Mus musculus will require the manufacture of a large living "database" of mutant mice over the next five years, with the ultimate goal of understanding comparable genes in humans.

The U.S. component of the multinational effort, the Knockout Mouse Project (KOMP), will target some 10,000 mouse genes, half the rodent's estimated complement. (Canadian and European researchers will tackle most of the rest.) Project investigators will have to make a lot of mice--or, more precisely, a lot of mouse embryos. Those will be used to derive embryonic stem cell lines, which can be turned back into embryos to make litters of live mice when they are needed for study. Grants issued last summer totaling nearly $50 million will go toward producing the first 8,500 or so of the cell lines, each carrying one disabled, or "knocked out," gene.