Terminal deoxynucleotidyltransferase: the story of an untemplated DNA polymerase capable of DNA bridging and templated synthesis across strands

•TdT has been known for more than 50 years as an untemplated DNA polymerase.•However, it was recently shown to be able to bind to a DNA synapsis in solution.•It was subsequently shown that TdT has in trans templated polymerase activity.•New crystal structures show TdT caught in the act of DNA synthesis across strands.•This dual activity of TdT can be understood in the context of its biological role.