N-6-(2-Deoxy-D-erythro-pentofuranosyl)-2,6-diamino-3,4-dihydro-4-oxo-5-N-methylformamidopyrimidine (MeFapy-dG) arises from N7-methylation of deoxyguanosine followed by imidazole ring opening. The lesion has been reported to persist in animal tissues. Previous in vitro replication bypass investigations of the MeFapy-dG adduct revealed predominant insertion of C opposite the lesion, dependent on the identity of the DNA polymerase (Pol) and the local sequence context. Here we report crystal structures of ternary Pol.DNA.dNTP complexes between MeFapy-dG-adducted DNA template:primer duplexes and the Y-family polymerases human Pol eta and P2 Pol IV (Dpo4) from Sulfolobus solfataricus. The structures of the hPol eta and Dpo4 complexes at the insertion and ex-tension stages, respectively, are representative of error-free replication, with MeFapy-dG in the anti conformation and forming Watson-Crick pairs with dCTP or dC.