Few quests have been as feverish as the search for alternative sources of taxol, the anticancer drug derived from yew trees. So far, the best candidates have not made an impact on the price tag. A single treatment cycle costs nearly $1,000 and may have to be repeated 10 times. Researchers from Montana report in Science that they have found the answer: a lowly tree fungus that produces the cancer-fighting chemical in minute quantities. If scientists can figure out how to boost the output, they should be able to create fermentation vats brimming with relatively inexpensive taxol.