A third study has found no evidence of a link between chronic fatigue syndrome, the debilitating condition whose cause is not known, and a virus called xenotropic murine leukemia virus–related virus (XMRV).

The link was first proposed in a study, published last October in Science, that found XMRV in blood cells of 67 percent of 101 chronic fatigue patients. But two other studies – one published in January in PLoS ONE, and the other in February in Retrovirology – found no trace of XMRV in two separate sets of chronic fatigue patients.

The third study, published today in the British Medical Journal, also found no evidence of XMRV in Dutch patients with or without chronic fatigue syndrome.

“Although our patient group was relatively small and we cannot formally rule out a role of XMRV, our data cast doubt on the claim that this virus is associated with chronic fatigue syndrome

in the majority of patients,” two of the study’s authors told Reuters.

Why the results of the original study have not held up is still somewhat of a mystery. In a commentary accompanying today’s report, two of the authors of the PLoS ONE study speculate that XMRV might still have caused the disease in the patients studied in the Science report, but might not have caused any other cases of the disease.

“The results from other US laboratories investigating XMRV and chronic fatigue syndrome are eagerly awaited,” write the authors of the commentary, Myra McClure of Imperial College London and Simon Wessely of King’s College London. “If the link fails to hold up, it will be another bitter disappointment to affected patients.”