The rapid amplification and isolation of specific DNA fragments made possible by the polymerase chain reaction (1,2) has become routine for a variety of molecular biology studies and applications. Taq DNA polymerase is still the most widely used enzyme for PCR amplifications, despite the fact that a number of other thermostable DNA polymerases are now available from commercial suppliers. Taq DNA polymerase is capable of catalyzing the addition of a single nucleotide, a deoxyadenosine (dA), to the 3′ ends of amplified PCR products (3), resulting in a single nucleotide 3′ overhang. This renders blunt-end ligation of such PCR products to cloning vectors inefficient.