Abstract: Abasic (AP) sites are among the most frequent endogenous lesions in DNA and present a strong block to replication. In Saccharomyces cerevisiae, an apn1 apn2 rad1 triple mutant is inviable because of its incapacity to repair AP sites and related 3'-blocked single-strand breaks (M. Guillet and S. Boiteux, EMBO J. 21:2833, 2002). Here, we investigated the origin of endogenous AP sites in yeast. Our results show that the deletion of the UNG1 gene encoding the uracil DNA glycosylase suppresses the lethality of the apn1 apn2 rad1 mutant. In contrast, inactivation of the MAG1, OGG1, or NTG1 and NTG2 genes encoding DNA glycosylases involved in the repair of alkylation or oxidation damages does not suppress lethality. Although viable, the apn1 apn2 rad1 ung1 mutant presents growth delay due to a G(2)/M checkpoint. These results point to uracil as a critical source of the formation of endogenous AP sites in DNA. Uracil can arise in DNA by cytosine deamination or by the incorporation of dUMP during replication. Here, we show that the overexpression of the DUT1 gene encoding the dUTP pyrophosphatase (Dut1) suppresses the lethality of the apn1 apn2 rad1 mutant. Therefore, this result points to the dUTP pool as an important source of the formation of endogenous AP sites in eukaryotes.