Recent studies have shown that pigs could donate hearts and kidneys for people. But livers, which rely on enzymes specific to each species, are more difficult. Now comes a promising new approach: partly humanize the pig's liver. Researchers at Ximerex in Omaha, Neb., infused human liver cells into fetal pigs and watched the cells produce human liver enzymes. But with no room in the pigs' livers, the human cells ended up in the spleen. To solve this space problem, Ximerex is developing genetically modified pigs programmed to kill off 20 to 80 percent of their own liver cells. The next step will be to transfer the pigs' immune cells to the patient to build up a tolerance of the hybrid organ. Working with the University of Nebraska, Ximerex hopes to begin human trials in 2005, says President William Beschorner.