Many oxidizing agents induce G-C to T-A and G-C to C-G transversions, and the frequency largely depends on the oxidative conditions. Guanine is the most oxidizable base among natural bases. The typical oxidative lesion product 8-oxoguanine (8-oxoG) is responsible for G-C to T-A transversion but not for G-C to C-G transversion, and 8-oxoG is more readily oxidized than guanine because of its lowered ionization potential. Recently, imidazolone (Iz), guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) have been demonstrated as oxidative lesion products of guanine and 8-oxoG, which could be responsible for G-C to C-G transversions by forming specific base pair formations.