PCR amplification of vaccinia DNA, Lister strain with the 16-plex Poxviridae primer set, with (lanes 6–10) or without (lanes 3 and 4) human DNA present in the reaction, for a reaction containing 4.8 mM MgSO4, 0.1 µM each primer and 2.7 pg (∼104 copies) of vaccinia Lister DNA, and using an annealing temperature of 43.9°C. Lane 3 shows specific amplification of vaccinia Lister DNA followed by the no template control in lane 4. Lanes 6–10 represent the experiments with mass ratios of vaccinia:human DNA of 1 : 1, 10 : 1, 100 : 1, 1000 : 1 and 10000 : 1, respectively. Lanes 12–14 represent amplification of human DNA only (no vaccinia present in reaction) for the same masses used at the 1 : 1, 100 : 1 and 10 000 : 1 ratios (or 2.7 pg, 0.027 pg and 2.7 × 10–4 pg human DNA, respectively). Lanes 5, 11 and 15 did not contain any sample. The arrow on the left points to the expected band (617 bp). The arrows on the right correspond to the 50-bp DNA ladder (lanes 1 and 16 contain the same ladder).