Deamination of DNA bases can occur spontaneously, generating highly mutagenic 
lesions such as uracil and hypoxanthine. In Escherichia coli two enzymes 
initiate repair at hypoxanthine residues in DNA. The alkylbase DNA glycosylase, 
AlkA, initiates repair by removal of the damaged base, whereas endonuclease V, 
Endo V, hydrolyses the second phosphodiester bond 3' to the lesion. We have 
identified and characterised a mouse cDNA with striking homology to the E.coli 
nfi gene, which also has significant similarities to motifs required for 
catalytic activity of the UvrC endonuclease. The 37-kDa mouse enzyme (mEndo V) 
incises the DNA strand at the second phosphodiester bond 3' to hypoxanthine- and 
uracil-containing nucleotides. The activity of mEndo V is elevated on 
single-stranded DNA substrate in vitro. Expression of the mouse protein in a DNA 
repair-deficient E.coli alkA nfi strain suppresses its spontaneous mutator 
phenotype. We suggest that mEndo V initiates an alternative excision repair 
pathway for hypoxanthine removal. It thus appears that mEndo V has properties 
overlapping the function of alkylbase DNA glycosylase (Aag) in repair of 
deaminated adenine, which to some extent could explain the absence of phenotypic 
abnormalities associated with Aag knockout in mice.