I know that others here are better informed about this subject than I am, but given how little we still know about XMRV, this discussion seems a little premature. From what little we know, I don't think we're going to be able to sensibly extrapolate whether XMRV and CFS is linked, or even likely to be linked - we're going to have to wait and see what the replication studies say. So far three against and not one in favour is looking a bit ominous, whatever the flaws there may be with the three negatives (you'd have thought we'd have heard something if others were finding the WPI's results easy to replicate). The WPI don't seem concerned, but we'll have to wait and see if their confidence is well founded. I really hope it's not a bust, but I don't think there's any point getting too commited to this link panning out yet.