Long-term co-culture assays were performed in 6 well tissue culture plates with inserts (TranswellH, Corning Inc.). This system uses a membrane with 0.4 mM diameter pores, which keeps the two cell types separated physically but simultaneously allows diffusion of nutrients and small molecules including virus particles of approximately 0.1 mM (including PERV). Each experiment was initiated with 75,000 PK-15 cells (insert) and 75,000 293T cells (well) as illustrated ( Figure 1A ). At 72 hr intervals, each cell type was washed with PBS, subjected to mild trypsinization and diluted into 4 parts fresh growth medium. Excess 293T cells were used to prepare genomic DNA (Qiagen DNeasy kit).