Gapped DNA synthesis assay with DNA pol β for dNTPs (dCTP, dATP, dTTP) and α,β-CXY analogues. The purity of the analogues was assessed from their failure to be inserted into a gapped DNA substrate. Primer (n) extension was assayed in the presence of low (L, 0.5 nM) or high (H, 50 nM) pol β and 1 mM analogue for 5 or 10 min, respectively. A: dCTP vs. 6; B: 3 vs. 11, 9, dATP and dTTP. The mobility of the extension product (n+1) with dCTP, dATP or dTTP serves as a reference, where most of the primer (200 nM) is extended after a 21 min incubation with 50 nM enzyme and 100 µM dNTP.