The insert is created by PCR using Taq polymerase. This polymerase lacks 3 to 5 proofreading activity and, with a high probability, adds a single, 3-adenine overhang to each end of the PCR product. It is best if the PCR primers have guanines at the 5 end as this maximizes probability of Taq DNA polymerase adding the terminal adenosine overhang. Thermostable polymerases containing extensive 3´ to 5´ exonuclease activity should not be used as they do not leave the 3´ adenine-overhangs.