Using the chemically synthesized oligonucleotides, we investigated the excision of the 3′-CTNAs in DNA by the human excision repair cross complementing protein 1-xeroderma pigmentosum group F (ERCC1-XPF) endonuclease, which is one of the main components of the nucleotide excision repair pathway. Here, ERCC1 is linked to xeroderma pigmentosum.