Could someone with a science or medical background or someone who has done much research on this explain why some scientist claim the NIH/FDA researchers found a different virus than the Whittmore Peterson researchers? As I understand, Whittmore Peterson found XMRV, the xenotropic version, transferred from human to human AND the FDA/NIH study found PMRV, the polytropic version, transferred from mouse to human. Some researchers say this is what you would expect to see from a retrovirus: different variants. This is what I am not understanding: Why didn't the NIH/FDA researchers find different variants of Xenotropic MuLv's? Shouldn't they have found variants of both Polytropic Murine Related Viruses AND variants of Xenotropic Murine Related Viruses. If not, why not? Where are the other Xenotropic viruses? Can you explain this to a person who knows very little about medical research in a way they would understand? Please and Thank You.