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. Manufacturers commonly sell TA Cloning "kits" with a wide range of prepared vectors that have already been linearized and tagged with an overhanging thymine.