nucleotidyl transferase is a component of the repair pathway for single nucleotide base excision repair. this repair mechanism begins when a single nucleotide is recognized by dna glycosylase as incorrectly matched or has been mutated in some way (uv light, chemical mutagen, etc.), and is removed. later, a nucleotidyl tranferase is used to fill in the gap with the correct base, using the template strand as the reference.