5-Hydroxymethyl-2'-deoxyuridine is a modified nucleoside, specifically a derivative of thymidine where the methyl group at the 5-position of the uracil base is replaced by a hydroxymethyl group. It is a structural analog of the canonical DNA nucleoside 2'-deoxyuridine. This compound is primarily significant in biochemical and biomedical research. It serves as a key metabolite and biomarker for the assessment of oxidative DNA damage. The formation of 5-hydroxymethyl-2'-deoxyuridine in DNA is associated with oxidative stress and certain pathological conditions. Consequently, it is widely used as a reference standard in analytical methods, such as liquid chromatography-mass spectrometry (LC-MS), for the quantification of oxidative DNA lesions in biological samples, aiding studies in aging, cancer, and other diseases linked to oxidative damage. As a modified nucleoside, it also finds application in nucleic acid chemistry research for investigating DNA repair mechanisms and the biological effects of oxidized DNA bases. Its use is confined to research settings as a biochemical tool and analytical standard.