Online Figure S1 ). In contrast, the control clones showed high levels of PERV transfer by co-culture day 17 and transmission events continued to accumulate through the duration of the experiment. The kinetics of PERV transmission were similar to those reported in Figure 1B (these results also contributed to the aforementioned transfer rate calculations). These data were further corroborated by additional experiments where PERV transmission was monitored simultaneously by Q-PCR and by reverse-transcriptase ELISA assays (Online Figure S2 ).