The crystal structures of DNA polymerase β in complex with two different mispairs (A-C and T-C) reveal that the two bases stack partially, rather than engage in hydrogen bonding with each other (Krahn et al., 2004, this issue of Structure). The N-domain of the polymerase, which is closed when the correct nucleotide pairs with the template base, adopts a partially open conformation suboptimal for catalysis.
Doublié, S. (2004). Infidelity out in the open. Structure. Cell Press. https://doi.org/10.1016/j.str.2004.09.002