Screening potential anticancer agents to find ones promising enough to make human clinical trials worthwhile has not been as straightforward as researchers would like. Not only have very few of the drugs that showed anticancer activity in animals carrying transplanted human tumors--known as xenografts--made it into the clinic, but a recent study conducted at the National Cancer Institute suggests that the tests are also missing drugs that do work in humans, possibly because animals and humans do not handle the drugs exactly the same way. And attempts to use human cells in culture don9t seem to be faring any better, partly because cell culture provides no information about whether a drug will make it to the tumor site. To create better models of cancer development in humans, investigators are now drawing on knowledge of human cancer-related gene mutations to genetically alter mice so that they carry the same kinds of changes that lead to cancer in humans. The hope is that the mice will develop tumors that behave the same way the human tumors do.