In the PNAS Early Edition this week, a trio of investigators at the National Institute of Environmental Health Sciences reports the crystal structure of a human DNA polymerase λ variant, which they say forms "a natural base-base mismatch with Watson-Crick-like geometry." This variant, the authors write, is "poised to misinsert dGTP opposite a template T," such that the resulting mismatch "has Watson-Crick geometry consistent with a tautomeric or ionized base pair, with the pH depende