Superposition of the active site of the ternary substrate complex of pol β with incoming 9, α,β-CF2-dTTP (gray carbons) and the corresponding ternary complex with 10, α,β-NH-dUTP (yellow atoms; pdb ID 2FMS). Active site carboxylates (D190, D192, D256) that coordinate two magnesium ions are shown as well as the 3’-OH of the primer terminus (P10). The fluorine atoms of 9 are cyan. Electron density for the 2Fo-Fc simulated annealing map (blue) is contoured to 5s illustrating the high quality of the electron density.