Researchers are looking for XMRV in CFS in the immune cells (T and B and others) in the blood. The papers indicated that the genetic sequences of the virus in T and B cells can get altered (or edited) over time by what are called APOBEC3 editing enzymes. Cells use these enzymes to attack retroviruses that have become integrated into our genome. Simply by exchanging the guanine bases in the retroviral sequences to adenine APOBEC3 stops the virus from replicating. Basically, except for the few virions that manage to escape this defense mechanism, T and B cells in the blood are a dead end for XMRV.
The XMRV in prostate cancer cells may be different, however, because the APOBEC3 enzyme is not found in those cells, thus allowing them to escape this editing process. Since XMRV was first discovered in prostate cancer cells it's not surprising that prostate cancer XMRV forms the basis for many of the PCR probes but it does mean studies using this form of the XMRV could miss the different looking virus in the T and B cells
