In January 6th, 2010edition of Science Now, Dr. Coffin writes in response to the UK article and the WPI response: “One distinct possibility, is that both papers are right. He called the PLoS ONE paper too "preliminary" to settle the debate and said XMRV could show more genetic variety, and thus be harder to detect, than anyone assumed. It's also possible that distinct strains of XMRV appear in different parts of the world, like the retroviruses HIV and HTLV (a leukemia virus).”
Coffin says “one more possibility, raised by many different scientists, is that CFS is actually a suite of diseases that presents the same symptoms and so might have many causes.” Lombardi (of the WPI) seconds this point. "It's naive to think that everyone with chronic fatigue has the same etiology. There's probably going to be a subset of people with CFS that have XMRV, and it will probably end up being classified as XMRV-related CFS."