DNA polymerase type-B epsilon subfamily catalytic domain. Three DNA-dependent DNA polymerases type B (alpha, delta, and epsilon) have been identified as essential for nuclear DNA replication in eukaryotes. DNA polymerase (Pol) epsilon has been proposed to play a role in elongation of the leading strand during DNA replication. Pol epsilon might also have a role in DNA repair. The structure of pol epsilon is characteristic of this family with the exception that it contains a large c-terminal domain with an unclear function. Phylogenetic analyses indicate that Pol epsilon is the ortholog to the archaeal Pol B3 rather than to Pol alpha, delta, or zeta. This might be because pol epsilon is ancestral to both archaea and eukaryotes DNA polymerases type B.