(D) Overlay of the primer terminus and active site with an undamaged reference structure (2PFO, black). The catalytic aspartic acids are shown in yellow, incoming dUMPNPP in magenta, and primer terminal dA in cyan. Expectedly, Mg2+ (neon) is occupying the metal B site. However, the metal A is absent in the structure, resulting in a non-catalytic position of the 3′-OH. This is consistent with other structures containing an incoming non-hydrolyzable analog. 