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.