An American study published in Science on 1st July found no evidence of XMRV or other MLVs in the blood samples of CFS people with an XMRV diagnosis. What they did find suggested that establishment of a successful MLV infection in humans would be unlikely. They also detected MLV sequences in commercial laboratory reagents which they thought showed the likelihood that previous evidence linking XMRV and MLVs to CFS was caused by laboratory contamination.

The results of another American study in the same journal suggests that the association of XMRV with human disease is due to contamination of human samples with virus originating from the recombination of 2 proviruses during tumor passaging in mice.

An article in Nature magazine asks whether these latest negative findings will have a detrimental effect on future research into CFS and reports that the authors of the original study which found XMRV have refused to retract their findings.