NTHL1 (hNTH1; endonuclease III-like protein 1) recognizes and binds dihydrouracil paired with guanine in the double strand DNA (Ikeda et al. 1998). 5,6-dihydrouracil is a form of DNA damage produced by ionizing radiation under anoxic conditions, so that cytosine is deaminated and C5-C6 double bond in the pyrimidine ring is saturated with hydrogen. 5,6-dihydrouracil mispairs with adenine, leading to G:C -> A:T transitions (Dizdaroglu et al. 1993).