When DNA is exposed to NO or HNO2, oxanine (Oxa) is formed as a major guanine lesion. For highly sensitive detection of Oxa using ARP, a probe molecule for DNA damage detection, the reactivity of ARP to Oxa was examined. Oxa site-specifically embedded in an oligonucleotide reacted with ARP but it took relatively long time (ca. 24 h) to completely convert Oxa to an ARP-labeled form.