Table 1. Model Systems.

enzyme	system	XRCa	dNpb	dNTP·dNtc	Ad	Bd
Polβ	Polβ1	2FMP	dC	dCTP·dG	Mg2+	Mg2+
	Polβ2	2PXI	dC	dCTP·dG	Mg2+	Mg2+
	Polβ3	2FMP	dC	dGTP·dC	Mg2+	Mg2+
	Polβ4	2PXI	dC	dGTP·dC	Mg2+	Mg2+
tPolλ	Polλ1	2PFO	dC	dCTP·dG	Mg2+	Mg2+
	Polλ2	2PFP	dC	dCTP·dG	Mg2+	Mg2+
tPolλΔL1	Polλ3	3PML	dC	dGTP·dC	Mg2+	Mg2+
	Polλ4	3PML	dC	dGTP·dT	Mg2+	Mg2+

^aXRC, X-ray crystal structure (PDB code).

^bdN_p, 3′-terminal nucleotide of the primer DNA strand.

^cdN_t, templating nucleotide of the template DNA strand.

^dA and B, active site metal ions (see Figure 2).