A critical part of the evidence linking XMRV to human disease is establishing that virus sequences isolated from patients are distinct from types that are found in mice and as cell culture contaminants. For the study described in the Science paper, XMRV sequences from CFS patients were compared with sequences from prostate tumors and endogenous proviruses mined from the genome of C57BL/6J mice. Using phylogenetic reconstruction algorithms, the genetic relatedness between the aligned viruses was visualized as a phylogenetic tree (see figure). The CFS XMRV sequences proved to be nearly identical to the prostate tumor XMRVs and distinct from other proviruses in the xenotropic MLV branch.
