DNA is continually damaged by endogenous and environmental agents leading to the formation of abasic (apurinic/apyrimidinic, AP) sites that are disruptive to DNA synthesis. Aldehyde Reactive Probe (ARP) is a biotinylated reagent for the detection and quantification of AP sites in damaged DNA. ARP reacts with aldehyde groups formed when reactive oxygen species depurinate DNA, resulting in covalent linkage of biotin to these AP sites. The biotin-tagged DNA can then be detected using common avidin-conjugated reporters such as avidin-HRP. The ARP method is highly sensitive, enabling detection of 2.4 AP sites per 1x107 nucleotides of DNA.