Whole cell protein extracts were prepared from 293T cells by suspending 500,000 cells in PBS, sonicating twice for 5 seconds and clarifying the lysates by centrifugation. Soluble protein levels were quantified using a BioRad Bradford assay. 10 mg of cell lysate was tested for reverse transcriptase activity using a C-type-RT activity assay (Cavidi Tech) following the manufacturers' instructions. Cell-free PK-15 supernatants (PERV-containing) were assayed directly using the Cavidi Tech ELISA assay.