DNA polymerase β comprises an aminoterminal 8-kDa domain and a carboxy-terminal 31-kDa domain. The N-terminal ssDNA binding domain has a deoxyribose phosphodiesterase activity while the C- terminal domain has a nucleotidyltransferase activity. Mammalian DNA polymerase β, a DNA repair polymerase, is constitutively expressed in cultured cells, but treatment of cells with the DNA-alkylating agents such as N-methyl-N'- nitro-N-nitrosoguanidine (MNNG) or methyl methanesulfonate up-regulates beta-pol level. DNA polymerase β fills single nucleotide gaps in DNA produced by the base excision repair pathway of mammalian cells.