XMRV is a newly identified human retrovirus that is similar to a group of mouse retroviruses (called murine leukemia viruses, or MLVs) scientists have known about for years. XMRV refers to xenotropic murine leukemia virus-related virus. It was first identified in 2006 in tissue samples from men with prostate cancer.

In a study published in the journal Science in October 2009, scientists reported a potential association of XMRV with chronic fatigue syndrome (CFS). In this study, XMRV was detected in approximately two-thirds of patients diagnosed with CFS. They also identified DNA of XMRV in the blood cells of some healthy persons and suggested a potential for XMRV transmission by transfusion or transplantation.

However, other recent studies, including a July 2010 research report from CDC scientists and colleagues at two other institutions, found no evidence of XMRV in CFS patients and in controls (see Updates).

More recently, investigators from the Food and Drug Administration (FDA), the National Institutes of Health (NIH), and Harvard Medical School published a report that presents evidence of MLVs in blood samples from CFS patients and healthy blood donors.  The authors state that although they found a broader group of MLVs, rather than XMRV, their results support the 2009 report in Science. The FDA /NIH paper was published online August 23, 2010, in the Proceedings of the National Academy of Sciences.

The reporting of different findings from different studies is not uncommon.  Various factors may have contributed to the differences in these studies, including selection criteria for inclusion of CFS patients, clinical complexities of CFS, and possible variations in XMRV and MLV infection rates among populations in different regions.  Moreover, XMRV is a recently discovered virus and much remains to be learned about this and  MLV-like viruses.  As additional studies are done, it is possible that new findings may emerge that differ from what has been previously reported.

The potential role of XMRV and MLVs in causing diseases such as prostate cancer and CFS remains unknown at this time. Additional research is needed to further evaluate a possible link of XMRV and MLVs with negative health outcomes, including prostate cancer and CFS. If it is determined that XMRV and MLVs may have a role in causing disease and illness, prevention recommendations can be made.

Although it is presumed that XMRV can be transmitted through blood transfusion, no such transmission event has been identified, and there is no known evidence of XMRV or MLV infection or related illness or disease in transfusion recipients. Agencies within the Department of Health and Human Services (HHS) are conducting studies to determine the prevalence of XMRV in the blood donor population.

CDC, FDA, and NIH investigators have been  collaborating with scientists from other agencies and groups with regard to XMRV and MLV research, and they will continue to do so.  HHS agencies will keep the public updated and informed as more information becomes available.