A number of people with ME report that they have different symptoms at different times during their illness. This is difficult to explain with any other hypothesis apart from the involvement of XMRV. XMRV could act as a gene hopper siliding about within the chromosomes of the same cell or different cells. Gammaretroviruses prefer to integrate within the start sequences of genes. So it is possible that they act as"puppet masters" for different abnormalities at different times leading to a particular pattern of symptoms at different times Here comes the science bit: Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called transposition. In the process, they can cause mutations and change the amount of DNA in the genome. Transposons were also once called jumping genes, and are examples of mobile genetic elements. There are a variety of mobile genetic elements, and they can be grouped based on their mechanism of transposition. Class II mobile genetic elements, or retrotransposons, copy themselves by first being transcribed to RNA, then reverse transcribed back to DNA by reverse transcriptase, and then being inserted at another position in the genome. transposons were also once called jumping genes,or"Junk DNA" Retrotransposons work by copying themselves and pasting copies back into the genome in multiple places. Initially retrotransposons copy themselves to RNA (transcription) but, in addition to being transcribed, the RNA is copied into DNA by a reverse transcriptase (often coded by the transposon itself) and inserted back into the genome. Retrotransposons behave very similarly to retroviruses, such as HIV. There are three main classes of retrotransposons: Viral: encode reverse transcriptase (to reverse transcribe RNA into DNA), have long terminal repeats (LTRs), similar to retroviruses It is only comparatively recently that retroviruses have been recognized as particularly specialized forms of eukaryotic transposons. In effect they are transposons which move via RNA intermediates that usually can leave the host cells and infect other cells. Or reintegrate in a different position in the DNA of the same cell The extra chromosomal linear DNA is the direct precursor of the integrated element and the insertion mechanism bears a strong similarity to "cut and paste" transposition.