Effect of exonuclease III on long PCR amplification performed with either rTth DNA polymerase alone (rTth) or in combination with Vent DNA polymerase (Vent). Aliquots of two phenol-extracted mouse liver DNA samples were used for long PCR amplification of the 8636 bp mtDNA fragment in the absence (exo 0) or presence of 25 U of exonuclease III (exo +). The 0.8% agarose gel was loaded with 22 µl of the PCR products. M, Hin dIII-digested phage λ DNA.