The SMUG1 gene codes for the enzyme single-strand selective monofunctional uracil DNA glycosylase. This enzyme can remove uracil from single-strand (unpaired) DNA. (Uracil can occur in DNA due to deamination of cytosine.)
What happens when this enzyme removes uracil? It seems what would be best is that a cytosine would replace the uracil in the DNA strand to restore the original DNA sequence, but I haven't found mention of this. The alternatives would seem to be that the enzyme simply splits the strand into two strands by removing the uracil (which are then unconnected since this is single-strand DNA), or it joins the two adjacent bases together, deleting the uracil but keeping the strand intact.