The REV3 and REV7 genes of the yeast Saccharomyces cerevisiae are required for DNA damage-induced mutagenesis. The Rev3 and Rev7 proteins were shown to form a complex with DNA polymerase activity. This polymerase replicated past a thymine-thymine cis-syn cyclobutane dimer, a lesion that normally severely inhibits replication, with an efficiency of similar to 10 percent. In contrast, bypass replication efficiency with yeast DNA polymerase a was no more than 1 percent. The Rev3-Rev7 complex is the sixth eukaryotic DNA polymerase to be described, and is therefore called DNA polymerase zeta.