But several other research teams in the last year have found no connection between chronic fatigue syndrome and these viruses, although none tried to replicate the exact methods used by researchers who reported an association.

The new papers in Retrovirology reported that contamination of tissue samples or other laboratory items with mouse DNA or viral genetic material could lead to false positive results for XMRV, and by extension other MLV-related viruses, specifically when using polymerase chain reaction technology. The technique rapidly produces millions of copies of genetic segments, so even minute traces of genetic contamination can skew results.

“Our conclusion is quite simple: XMRV is not the cause of chronic fatigue syndrome,” said the senior author of one of the studies, Greg Towers, a professor of virology at University College London, in a statement released by Wellcome Trust Sanger Institute, the British research center that co-sponsored it.

Other scientists and advocates for patients have sharply criticized such certainty as unwarranted, noting that the Retrovirology papers themselves expressed their findings in more cautious terms. The critics agree that contamination can be a serious issue when using polymerase chain reaction technology. But the new papers, said Eric Gordon, a doctor in Santa Rosa, Calif., who treats many patients with the illness, do not evaluate other strategies besides P.C.R., as the technique is known, for detecting the MLV-related viruses, like testing for an immune response and culturing the viruses in cell lines.

“The articles make the point that P.C.R. doesn’t work that well for these viruses, and then they act like that disproves the whole idea,” said Dr. Gordon.

XMRV was first identified in 2006 and has been detected in prostate cancer patients in some studies. It was linked to chronic fatigue syndrome in October 2009 in a paper in the journal Science by researchers from the Whittemore Peterson Institute for Neuro-Immune Disease at the University of Nevada, Reno, the National Cancer Institute and the Cleveland Clinic.

The researchers relied on P.C.R. technology to show that about two-thirds of patients but less than 4 percent of control subjects harbored XMRV. Using other technologies, however, they also documented an antibody response in some chronic fatigue syndrome patients, and reported that XMRV in human blood could infect other human cell lines.