although there are many repair pathways in cells, some lesions still escape repair inevitably and remain in genome. In cells, the molecular mechanism of translesion DNA synthesis has been one of the major unsolved problems in DNA repair for a long time. Recently, it was found that the members of a structurally related UmuC/DinB protein superfamily have DNA polymerase function. Unlike the classical replicative DNA polymerases, these newly identified DNA polymerases can carry out translesion DNA synthesis in both error-prone/mutagenic and/or error-free ways. It was also found that their functions are conserved from bacteria to human.