ARP (Aldehyde reactive probe) is a chemical compound that serves as an aldehyde-reactive probe by specifically tagging AP sites in DNA with biotin residues. It is used to detect abasic sites, which are common forms of DNA damage and intermediates in mutagenesis and carcinogenesis. ARP is highly sensitive, capable of detecting abasic sites at the femtomolar level (fewer than 1 site per 10^4 nucleotides).