Description: Two Bifunctional polynucleotide phosphatase/kinase (PNKP) molecules interact with two FHA-binding motifs in DNA repair protein XRCC1 (XRCC1) upon phosphorylation of T519 and T523 in the motifs. Additional phosphorylation of S518 and S525 further increases the affinity of the interaction. S518 and T523 are consensus CK2 phosphorylation sites, T519 and S525 atypical.