LAST October a discovery was made that brought hope to millions of sufferers from chronic fatigue syndrome (CFS). A group of researchers found a bug with the long-winded name of xenotropic murine leukaemia virus-related virus (XMRV) in 67% of American patients with CFS (as opposed to 4% of healthy controls). This figure increased dramatically when the patients were retested.

The news was exciting for patients because CFS is a debilitating disorder of long-term tiredness for which there is no simple explanation, and certainly no sniff of a cure. It has even attracted a certain degree of media scepticism—being dubbed, at one time, “yuppie flu”.

XMRV is certainly a reasonable candidate to cause CFS. It has also been implicated in prostate cancer, breast cancer and lymphoma. Last year's study, published in Science by Judy Mikovits of the Whittemore Peterson Institute in Reno, Nevada, and her colleagues, caused such excitement that it quickly led to commercial testing for XMRV infection. Furthermore, because XMRV is a retrovirus, like HIV, several preclinical studies have been looking at whether antiretroviral drugs might have an effect on it. One of these studies has found that of the ten licensed compounds tested, only AZT inhibited replication of XMRV.

The new year, though, has brought new complications, in the form of a study that has been unable to replicate Dr Mikovits's work. Writing in the Public Library of Science, Otto Erlwein of Imperial College, London, and his colleagues report that they are unable to find XMRV in any of almost 200 CFS patients in Britain.

One possible explanation is that one of the two groups of scientists made a mistake in their testing, something that both are adamant is not the case. As scientific punch-ups go, this is shaping up to be a good one, with blows (albeit polite ones) being thrown across the Atlantic.

The Americans say that the British were not looking for DNA directly in white blood cells, and by using whole blood they diluted their samples too much. Kerpow! They add that the design of the British experiment did not prove that it could actually pick up XMRV in patients. Zapp! Finally, they say the British must prove they can actually detect XMRV in one of the American samples, something that they would have been delighted to provide had they been asked. Biff!

For their part, the British insist their controls are just fine and hint that the American laboratory may have had contamination problems. Blam! They also add that a number of other labs are about to publish results of similar studies looking for XMRV and say they are “very confident” about their findings. Splat!

Frustrating though this may be for sufferers from CFS, it is discussions like these, as one group of researchers tries to replicate the results of another, that lead to scientific progress. In the meantime, one of the companies licensed to do the American XMRV test claims that of the 300 patients, including some from Britain, who have sent samples in for testing, 36% have tested positive for XMRV. It seems likely that the causes of CFS will continue to be a subject of controversy for a while yet.