A new retrovirus, xenotropic murine leukaemia virus-related virus (XMRV), first identified in tumor tissue of individuals with prostate cancer, was subsequently found in 68 of 101 US patients with chronic fatigue syndrome (CFS). XMRV was not detected in blood samples of 186 confirmed CFS patients in the United Kingdom. A second independent study in the UK (pdf) has also failed to reveal XMRV in CFS patients.