It is known that formylation of thymine base induces purine transition in DNA replication. In order to establish the structural basis for such mutagenesis, crystal structures of two kinds of DNA dodecamers d(CGCGRATf5UCGCG) with f5U = 2'-deoxy-5-formyluridine and R = A or G have been determined. The f5U residues form a Watson-Crick-type pair with A and two types of pairs (wobble and reversed wobble) with G, the latter being the first example. Structural modeling suggests that the DNA polymerase can accept the reversed wobble pair with G, as well as the Watson-Crick pair with A.