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We have shown that DNA polymerase β, the only nuclear DNA polymerase present in adult neurons, cannot discriminate between dTTP and dUTP, having the same K_m for both substrates. This fact suggests that during reparative DNA synthesis, in adult neurons, dUMP residues can be incorporated into DNA. Since uracil DNA-glycosylase functions to prevent the mutagenic effects of uracil in DNA coming as a product of deamination of cytosine residues or as a result of dUMP incorporation by DNA polymerase, we have studied the perinatal activity of uracil DNA-glycosylase and of 2 enzymes (nucleoside diphosphokinase and dUTPase) involved in dUTP metabolism. Our data indicate that during neuronal development there is a rapid decrease in uracil DNA-glycosylase which could impair the removal of uracil present in DNA in adult neurons. However, misincorporation of dUMP into DNA might be kept to a low …