Hi Hope, However, to date (unless something happened within the last year) no one has proven that any HERV is capable of producing infectious particles. Yet the WPI/NCI study showed that XMRV could infect previously uninfected cells. Some replication-deficient/incomplete HERV particles can bud, but that doesn't explain the infectivity of the WPI samples, nor a number of other findings of that study. Technically an ultrastructural EM study of the particles in question should be able to make clearer whether they are complete virions or incomplete particles that could be from HERVs. But the stronger argument against contamination stems from a series of findings in the Lombardi study, which relegate the contamination argument to extreme improbability, and for which there has been no explanation offered by those who raise contamination as an issue.