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. The target vector is linearized and cut with a blunt-end restriction enzyme. This vector is then tailed with dideoxythymidine triphosphate (ddTTP) using terminal transferase. It is important to use ddTTP to ensure the addition of only one T residue. This tailing leaves the vector with a single 3'-overhanging thymine residue on each blunt end. "Blunt end" TOPO cloning Polymerases (such as Phusion) or restriction enzymes that produce blunt ends can also be used for TOPO cloning. Rather than relying on sticky ends, the blunt end TOPO vector has blunt ends where the topoisomerase molecules are bound. Commercial kits, such as the Zero Blunt® Cloning Kit from Invitrogen, are available. References Cloning Molecular biology