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Telomere degradation is reported to be the initial event that triggers multiple DNA damage responses, and the nature of the event is likely to be the disclosure of concealed single-stranded 3' overhang. Exogenous telomere homolog oligonucleotide can mimic the signal of telomere degradation in the absence of UV-induced DNA damage, and enhance the DNA repair capacity of organisms, so as to defend against the damage induced by UV irradiation. The fact that telomere homolog oligonucleotides trigger SOS-like responses and cause selective apoptosis of malignantly transformed cells may provide an important clue to the prevention and treatment of photo-induced skin tumors.