XMRV was originally described in 2006 in the tumour tissue of patients with a familial form of prostate cancer associated with mutations that impair the function of the antiviral defence protein RNase L . Three independent groups subsequently reported the presence of XMRV in a significant proportion of prostate cancers, but the linkage to polymorphisms of the RNase L gene was not confirmed. In contrast, at least seven other studies have reported an inability to detect, or extremely low prevalence of, XMRV in prostate cancer despite using highly sensitive PCR-based assays.
Immunohistological, in situ-hybridisation and serological studies have also been inconsistent in their findings. Some studies [1, 2], using immunostaining and/or FISH, detected XMRV in a small percentage of stromal cells but not in tumour cells, whereas others using similar techniques reported XMRV predominantly in tumour cells rather than stromal cells. In a recent study, Aloia and colleagues  employed HPLC purified proteins to raise the antisera used for immunostaining, and were unable to find any trace of XMRV at all in nearly 800 prostate tumours analysed. They suggested that the positive immunostaining described in earlier studies may have been due to the use of non-specific antisera exhibiting cross-reactivity with human cellular proteins .
Similar controversy surrounds claims of an association between XMRV and chronic fatigue syndrome (CFS). In a highly publicised study, Lombardi and colleagues detected XMRV in 67% of CFS patients and 3.7% of healthy controls by nested PCR . Since Lombardi's initial publication, there have been numerous attempts by other groups in several countries to confirm the linkage between XMRV infection and CFS; but as yet none have succeeded. Curiously, one study described PCR detection of a second MLV (modified polytropic MLV), but not XMRV itself, in the blood of some CFS patients . XMRV has also been sought in a variety of other diseases including amyotrophic lateral sclerosis, multiple sclerosis, autism, immunosuppression, rheumatoid arthritis, fibromyalgia and paediatric idiopathic disease; but all with negative results.
A number of recent publications have attempted to explain these confusing and highly contradictory reports by calling attention to the significant risk of false positive XMRV results due to laboratory contamination, and to PCR contamination in particular. The considerable potential for false positives arising from minute traces of murine DNA contaminating test samples or reagents has been clearly demonstrated as has the risk of erroneous results due to contamination from human tumour cell lines infected with XMRV (e.g. 22Rv1) or other xenotropic MLVs acquired by xenografting in mice .