http://www.ncbi.nlm.nih.gov/m/pubmed/22031947/ Characterization, mapping, and distribution of the two XMRV parental proviruses. Cingz O, et al. were unable to detect integration sites for preXMRV1 in either Hsd or NU/NU mice. In addition only 40% of NU/NU mice contained any evidence of preXMRV-1 itself, says a new paper published in January's Journal of Virology. From the findings in the above paper, it appears that Paprotka et al failed to screen any wild mice for any viruses which could be the source of murine related retroviruses, so the assumption they make is that any MRV provirus which does not have a 24 nt deletion cannot be a MRV provirus is like saying that any HIV provirus not containing a single deletion sequence is not HIV, yet deletion mutants of HIV are commonplace and no-one has proved that the same is not the case in other retroviruses. The failure to find any evidence that preXMRV1 is integrated into the genome of mice that are alleged to have been used in the formation of the 22rv1 cell line, together with the small number of NU/NU mice found to contain pre xmrv1 sequences, strongly suggests that preXMRV1 is a PCR contaminant. Dr John Coffin detected preXMRV1 integrated into the DNA of a strain of mouse supplied by Greg Towers who works with the Welcome Trust. Perhaps this is the source?