Additional clarification can be found at X Marks the Spot? from today's Wall Street Journal, stating in part, "Researchers led by NIH infectious diseases specialist Harvey Alter found a family of retroviruses called MLV-related viruses in a majority of CFS patients. X[MRV] is a member of that family. But the Alter team found that the strains of virus in the patients that they tested were closer to another branch of the same family — viruses known as polytropic MLVs (you got it — shortened to “P”) — than to X."
The above article later states that CFS may turn out to be "a constellation of diseases, only a proportion of which will be shown to be associated with XMRV/MLV." Another possibility is that "CFS could end up looking more like hepatitis...in that patients have similar symptoms that are caused by five distinct viruses." AS WPI has continued studying their samples since the October Science publication they have found "almost all of them are positive for one or more MLV-related viruses, including X and P."