The rate of PERV transfer was calculated using the pol gene levels from the last two data points (usually spanning a 3 day period). The difference between these levels represents PERV pol gene DNA that has accumulated per 100,000 human beta-actin gene copies (50,000 cells assuming that the 293T cell line has two beta-actin copies) per time period. Individual rates from 5 independent experiments were averaged to determine the overall transmission rate (190+/262 events per day per 50,000 cells). Data from Figures 1B, 2B and S2A contributed to rate calculations.