(A) Scheme of primer-template substrate with a sequence forming a parallel G4 structure on the template strand ahead of the primer that can be extended by Pol δ. Asterisk indicates 5´-FAM label. Numbers indicate lengths of primers and gaps in nucleotides (nt). (B) Time-resolved primer extension assay with 10 nM of Pol δ alone (–) and in the presence of 35 nM FANCJ wild-type or variants. Numbers indicate the mean percentage of full extension and standard deviations from two independent experiments. (C,D) Primer extension assay with 10 nM of Pol δ and increasing amounts of different FANCJ variants (C) and quantification (D). In the graph, mean values and standard deviations of two independent experiments are depicted. N+12 denotes position of G4 block relative to 3´-end of primer. S, DNA substrate without protein; WT, wild-type; KR, K52R; CS, C283S; CR, C283R; MI, M299I; LF, L340F; AP, A349P.