The human UNG-gene encodes nuclear (UNG2) and mitochondrial (UNG1) forms of uracil-DNA glycosylase using differentially regulated promoters, PA and PB, and alternative splicing to produce two proteins with unique N-terminal sorting sequences. Uracil in DNA may result from deamination of cytosine which can give rise to transition mutations. Uracil-DNA glycosylase is the DNA repair enzyme responsible for the removal of uracil from DNA. This is probably a biologically important function in the prevention of mutagenesis resulting from cytosine deamination. UNG2 removes misincorporated dUMP residues.