Pyrimidine biosynthesis enzymes function in many cellular processes and are closely associated with pyrimidine antagonists used in cancer chemo-therapy. These enzymes are well characterized from bacteria to mammals, but not in a simple metazoan. To study the pyrimidine biosynthesis path-way inCaenorhabditis elegans, we screened for mutants exhibiting resis-tance to the anticancer drug 5-fluorouracil (5-FU).