XMRV is a human retrovirus discovered in cancerous prostate tissues in 2006 by Dr. Robert Silverman, a cancer biologist at the Cleveland Clinic. In October, 2009, Dr. Silverman along with other researchers reported finding the virus in the majority of patients they studied who have Chronic Fatigue Syndrome. Studies in the Netherlands and the United Kingdom were unable to replicate their findings.

XMRV is a descriptive name:
X - Xenotropic (a virus that is found benignly in cells of one animal species but only infects different species)
M - Murine leukemia, the parent virus
RV - Related Virus