The double proton transfer reaction in the guanine-cytosine (GC) base pair is studied, using density functional theory, to understand the chances of mutations under the effect of uniform electric fields in the order of 10(8) to 10(9) V m(-1). On the basis of potential energy surfaces, reaction Gibbs energies, equilibrium constants, imaginary frequencies, forward and reverse barrier heights, tunneling-corrected rate constants, half-lives of the forward and reverse reactions, percent tautomerization, and Boltzmann distributions, it was found that fields >=+3.60 x 10(9) V m(-1) facilitate the mutation in the GC base pair and reduce the rectification of point mutations. Fields applied along the double proton transfer in the -x (defined in the C to G direction) direction favor the canonical over the rare tautomers. Tunneling-corrected rate constants of the forward reaction increase exponentially with stronger fields in the -x direction and follow a Gaussian curve for the reverse reaction.