7002 Nucleic Acids Research, 2008, Vol. 36, No. 22 Figure 1. (A) Analogs of G and C evaluated in this study; (B) Primer extension products prepared using Phi29 N62D DNA polymerase in the presence of regular and base-modiﬁed dNTPs. Speciﬁcity of pairing was determined by hybridizing gel-puriﬁed 26-mers to perfect-match and mismatch probes under stringent conditions; (C) Base pairing in nA–sT, sG–C and G–sC couples is weakened by steric clash between a thioketo group on one base and an amino group on the other. In our screening of dNTP analogs we used the N62D mutant of f29 DNAP. Table 1 summarizes the yields of single-strand (SS37) and hairpin (HP25) primer extension products (Figure 1) synthesized with diﬀerent analogs of dGTP or dCTP. This table also contains the results for a parallel set of reactions that also contained dnATP and dsTTP in place of dATP and dTTP. Anticipating the probable use of