While mammalian DNA polymerase beta (Pol beta), which is a member of the Pol X family, play important roles in base excision repair (BER) that efficiently removes DNA base lesions arising from both endogenous and exogenous agents, this protein has been found only a subset of animals. To understand natural evolution of this enzyme, we isolated and characterized Pol beta from jellyfish Aurelia sp.1. (AsPol beta). Despite of phylogenetic distance and environmental differences between jellyfish and mammals, in vitro assays showed biochemical characteristics of AsPol beta were very similar to those of a mammalian counterpart. We also searched two other homologs of mammalian genes that were involved in short patch (sp) BER in the nucleotide sequence database, and found that both of these homologs were encoded in the genomes of a lineage from Cnidarians through mammals and Arthropods. This study suggests that a DNA repair mechanism resembling mammalian sp-BER may be largely limited to a subset of animals. On the basis of our findings and previous reports, we discuss possible evolutional model of Pol beta and the other members of the Pol X family. (C) 2011 Elsevier Inc. All rights reserved.