No Xenotropic Murine Leukemia Virus–related Virus Detected in Fibromyalgia Patients

To the Editor: Xenotropic murine leukemia virus-related virus (XMRV) is a recently described human retrovirus that has been associated with prostate cancer and chronic fatigue syndrome (CFS) (1,2). XMRV is similar to a classic murine endogenous leukemia retrovirus, murine leukemia virus (MLV), which infects strains of mice that do not express the specifi c viral receptor. XMRV is genetically close to, although differentiable from, MLV. The fi rst evidence of its presence in humans was obtained by Urisman et al. in prostate cancer tissue (1). In 2009, Lombardi et al. ( This association was notable because XMRV sequences were found in only 4% of healthy controls. These results have generated controversy because several independent studies, mainly in Europe (3-5) but also in North America (6), have been unable to detect XMRV sequences in patients with CFS. Furthermore, a recent report from North America (7) appears to confi rm the initial results by Lombardi et al. (2) in patients with CFS and expands the viral association to a wider variety of XMLV-related viruses that seem closer to polytropic mouse endogenous retroviruses.