Genomic DNA was isolated from human 293T cells using the DNeasy kit (Qiagen). Duplicate 25 ml PCR reactions consisting of 10 ng of 293T genomic DNA, 100 nM primers and 26 iQ SYBR Green super mix (BioRad) were run on an iCycler iQ Multicolor Real-Time PCR detection System (BioRad). The thermocycler conditions consisted of an initial denaturation of 95uC for 5 min and 50 cycles of denaturation (95uC for 15 sec) and annealing (58uC for 30 sec). After the 50 cycles, a melting curve analysis (55uC to 95uC) was performed to confirm product specificity. The cycle threshold (C T ) was generated using BioRad software and it was used to calculate the amount of target DNA (PERV pol or human beta-actin). A standard curve was generated using the method of Dorak [56] and a dilution series (10 to 10 7 copies) of a linearized plasmid containing the relevant 193 bp PERV pol gene fragment. The equation generated from the standard curve (slope and y intercept) was used to determine the efficiency of the PCR reaction and to quantify the number of PERV pol gene or human beta-actin copies in the Q-PCR reactions. PERV copy numbers were normalized to those of beta-actin using the method of [56] . The primer sets used in this study were: PERV pol (193 bp):  59-AAC CCT TTA CCC TTT ATG TGG AT and 59-AAA GTC  AAT TTG TCA GCG TCC TT; 