Exonuclease III enhances long PCR amplification from phenol-extracted DNA samples. DNA samples were extracted with phenol/chloroform and either stored at –20 or –80°C for several years (mouse and human DNA, respectively) or used immediately (rat DNA). After PCR, agarose gels (0.7–1.2%) were loaded with 22 µl of the PCR products together with Hin dIII-digested phage λ DNA (M). ( A ) Five mouse liver DNA samples (ML1–ML5) were used for PCR co-amplification of the 316 and 8636 bp mtDNA fragments, using Protocol 1a without (exo 0) or with 25 U of exonuclease III (exo +). ( B ) Four rat liver DNA samples (RL1–RL4) were used for long PCR amplification of a 15.4 kb mtDNA fragment, using Protocol 2 without (exo 0) or with 25 U of exonuclease III (exo +). ( C ) Five human blood DNA samples (HB1–HB5) were used for long PCR amplification of a 5 kb fragment from the human CYP2D6 nuclear gene, using Protocol 3 without (exo 0) or with 50 U of exonuclease III (exo +).