DNA polymerase protein structures. The figures were generated with the free software Rasmol (RasWin Molecular Graphics, Windows Version 2.4) and files extracted from the Protein Data Bank (PDB) . (a) Bacteriophage T7 DNA polymerase (type A; PDB code: 1t7p) . Blue, catalytic domain; green, 3'-to-5' exonuclease domain. (b) Thermococcus gorgorianus type B DNA polymerase  (PDB code: 1tgo). Blue, catalytic domain; green, 3'-to-5' exonuclease domain; orange, amino-terminal domain; yellow, linker region.