Deoxyuridine triphosphate nucleotidohydrolase (dUTPase) is a ubiquitous enzyme that functions in the hydrolysis of dUTP to dUMP and pyrophosphate. Canman and co-workers have demonstrated that, in certain human tumor cell lines, increased levels of dUTPase are responsible for an increase in resistance to the cancer chemotherapeutic agent fluorodeoxyuridine (FUdR), a thymidine synthase inhibitor. Two distinct forms of dUTPase exist in humans. Cellular fractionation experiments suggest that the more abundant, lower mass form of dUTPase (DUT-N) localizes in the nucleus, while the higher mass form (DUT-M) is associated with the mitochondria.