Competitive binding assay of D210N-APE1 to DNA containing either (A) coumarin:THF or (B) guanine:THF. Lane 1 shows the labeled DNA in the absence of the protein. Lane 2 shows the protein binding in the absence of competitor (18% A, 98% B). Lane 3 shows binding in the presence of 100-fold excess of unlabeled, specific oligodeoxynucleotide duplex that has THF opposite coumarin (5%, A) or guanine (3%, B). Lane 4 shows the binding in the presence of 100-fold excess of unlabeled, nonspecific competitor DNA that has guanine opposite cytosine (22%, A) (97%, B).