To determine whether expression of human APOBEC3G would inhibit the transfer of PERV from pig to human cells it was first necessary to establish a long-term co-culture system. A trans-well assay was set up to monitor PERV transmission from pig kidney PK-15 fibroblasts to recipient human embryonic kidney 293T cells ( Figure 1A ). These two cell types were used because transmission from PK-15 to 293 cells had been reported previously [11] . The trans-well system enabled co-cultures to be sustained for several weeks, and it facilitated the recovery of each cell type for downstream analyses. An additional benefit of this co-culture system (not provided by transient assays) is that it enables the simultaneous analysis of multiple, endogenous PERV elements, which are precisely the targets one would want to monitor and ideally inhibit in (xeno)transplantation procedures.