take home message XMRV will not replicate in non replicating blood cells.guess what type of blood cells the European studies used for their PCR runs? Retroviruses as Carcinogens and Pathogens: Expectations and Reality By Peter H. Duesberg Cancer Research, Vol. 47, pp. 1199-1220, (Perspectives in Cancer Research), March 1, 1987. The vast majority of the tumor viruses are retroviruses and DNA viruses that do not contain oncgenes. The RNA genomes of all retroviruses without oncgenes measure only 8 to 9 kilobases (13, 22). They all encode three major essential genes which virtually exhaust their coding capacity. These are in the 5' to 3' map order gag which encodes the viral core protein, pol which encodes the reverse transcriptase, and env which encodes the envelope glycoprotein (23, 24). Although these viruses lack oncgenes they are considered tumor viruses, because they were originally isolated from tumors and because experimental infections may induce tumors under certain conditions. However, in contrast to tumors caused by viruses with oncgenes, such tumors are always monoclonal and induced reproducibly only in genetically selected animals inoculated as newborns after latent periods of over 6 months (see below). Because of the long latent periods, these retroviruses are said to be "slow" viruses (13, 16), although their mechanism of replication is exactly the same as that of their fast and efficient relatives with oncgenes that transform cells as soon as they infect them (5, 19) (Table 1). The retroviruses are also considered to be plausible natural carcinogens because they are not cytocidal and hence compatible with neoplastic growth and other slow diseases. Indeed, retroviruses are the only viruses that depend on mitosis for replication (13, 25). # Weiss, R., Teich, N., Varmus, H., and Coffin, J. RNA Tumor Viruses. Ed. 2. Cold Spring Harbor, NY: Cold Spring Harbor Press, 1985. Rubin, H., and Temin, H.M. A radiological study of cell-virus interaction in the Rous sarcoma. Virology, 7: 75-91, 1958.