One of my research interests currently is the isolation, characterization, and study of human retroviruses in disease. The discovery of XMRV in CFS/ME represents just a small step. Sufferers of neuroimmune diseases have been searching for answers for decades - without any explanation. While it is still too early to tell if the results of the Whittemore Peterson institute will be replicated, it is the objective of most groups working in good faith towards this goal to do so. If consistent results are reported on a worldwide basis, it's a huge victory. Even if we see results which show a regional consistency - it still means something, and we have to keep looking further - hence a possible explanation for the difference in the findings of DeFreitas.
In such a case, I entertain the possibility that there maybe a handful of related retroviruses capable of inciting symptoms of ME/CFS, much like there are many viruses capable of producing cold like symptoms. This is a research proposal that I have put forth to a group of academic institutions here in the UK, and I hope to have an answer soon. At the least, I would not be surprised to identify multiple strains of XMRV on a regional basis throughout the world. It would also be interesting to attempt to replicate the Defreitas study.
At this stage, I don't think it would be prudent to let a retroviral cause for neuroimmune disease slip us by once again. I am for without a doubt certain that we have the cause of these disorders within sight - and we would be foolish not to think so. We have large libraries of retroviral genomes to screen from, making the identification of closely related viruses readily possible - just like we have HIV-1 and HIV-2. Over the years, we've accumulated evidence of a virus, and ME/CFS sufferers must be given answers, so all leads must be pursued to the end.