Organisms require faithful DNA replication to avoid deleterious mutations. In yeast, replicative leading-and lagging-strand DNA polymerases (Pols and ␦, respectively) have intrinsic proofreading exonucleases that cooperate with each other and mismatch repair to limit spontaneous mutation to less than 1 per genome per cell division. The relationship of these pathways in mammals and their functions in vivo are unknown. Here we show that mouse Pol and ␦ proofreading suppress discrete mutator and cancer phenotypes. We found that inactivation of Pol proofreading elevates basesubstitution mutations and accelerates a unique spectrum of spontaneous cancers; the types of tumors are entirely different from those triggered by loss of Pol ␦ proofreading. Intercrosses of Pol -, Pol ␦-, and mismatch repair-mutant mice show that Pol and ␦ proofreading act in parallel pathways to prevent spontaneous mutation and cancer. These findings distinguish Pol and ␦ functions in vivo and reveal tissue-specific requirements for DNA replication fidelity.DNA replication ͉ genetic instability ͉ DNA polymerase fidelity ͉ mismatch repair

Mutations are a hallmark of cancer. Normal cells minimize spontaneous mutations through the combined actions of polymerase base selectivity, 3 3 5 exonucleolytic proofreading, mismatch correction, and DNA damage repair. To determine the consequences of defective proofreading in mammals, we created mice with a point mutation (D400A) in the proofreading domain of DNA polymerase ␦ (pol␦, encoded by the Pold1 gene). We show that this mutation inactivates the 3 3 5 exonuclease of pol␦ and causes a mutator and cancer phenotype in a recessive manner. By 18 months of age, 94% of homozygous Pold1 D400A/D400A mice developed cancer and died (median survival ‫؍‬ 10 months). In contrast, only 3-4% of Pold1 ؉/D400A and Pold1 ؉/؉ mice developed cancer in this time frame. Of the 66 tumors arising in 49 Pold1 D400A/D400A mice, 40 were epithelial in origin (carcinomas), 24 were mesenchymal (lymphomas and sarcomas), and two were composite (teratomas); one-third of these animals developed tumors in more than one tissue. Skin squamous cell carcinoma was the most common tumor type, occurring in 60% of all Pold1 D400A/D400A mice and in 90% of those surviving beyond 8 months of age. These data show that pol␦ proofreading suppresses spontaneous tumor development and strongly suggest that unrepaired DNA polymerase errors contribute to carcinogenesis. Mice deficient in pol␦ proofreading provide a tractable model to study mechanisms of epithelial tumorigenesis initiated by a mutator phenotype.

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Women's responses highlight the significant impact of the experience on their lives and illuminate to primary care providers the importance of assessing, recognizing, and appropriately addressing their specific concerns.