5,6-Dihydroxy-5,6-dihydrothymine (Thymine glycol, BA0198) is formed when thymine undergoes oxidative stress, including ionizing radiation. Oxidation of the 5,6 double bond generates two chiral centers at C5 and C6, with the cis-5R,6S form as the predominant product over the cis-5S,6R form. The presence of thymidine glycol in natural DNA has biological consequences and many organisms possess specific repair enzymes for the excision of this lesion.