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DNA polymerase type-B delta 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. Presently, no direct data is available regarding the strand specificity of DNA polymerase during DNA replication in vivo. However, mutation analysis supports the hypothesis that DNA polymerase delta is the enzyme responsible for both elongation and maturation of Okazaki fragments on the lagging strand.