3 pmol) was annealed with the unlabeled oligonucleotide (6 pmol) in a reaction mixture (50 μl) containing 20 mM Tris–HCl (pH 7.5), 10 mM MgCl2 and 25 mM NaCl, which were heated to 95°C, kept at 67°C for 1 h, and then