About one million Americans carry the hepatitis B virus (HBV) which can lead to liver cancer. But the search for treatments has been stymied by the lack of a suitable animal for testing new drugs. Rodents don't get HBV, and chimpanzees, which do, are expensive.

XTL Biopharmacueticals in Rehovot, Israel, has come up with a solution by creating a mouse with a bit of human liver inside it. XTL's recipe: destroy the immune system of a normal mouse with gamma rays. Next, transplant a peppercorn-sized fragment of HBV-infected human liver, along with bone marrow from an immune-deficient mouse. XTL calls the resulting creature a "trimera" because it has components of three different animals. Since the mouse's weak immune system doesn't reject the human cells, there is time to test the effects of anti-viral drugs. XTL hopes to develop trimera models for cancer.