Dihydro dU (5,6-DHU) is primarily used in studies of irradiative DNA damage and associated repair mechanisms. In the cell, 5,6-DHU DNA lesions are formed by gamma irradiation of deoxycytosine under anoxic conditions, resulting in deamination followed by addition of hydrogen at C5 and C6 of the base. DHU is highly mutagenic, leading to C-to-T transitions at the mutation site (because DNA polymerase inserts A opposite the 5,6-DHU lesion) (1). Because DHU is recognized and removed by endonuclease III and other eukaryotic endo III homologs, DHU-modified oligos are used in model systems for studying DNA damage and repair mechanisms.