Running the gamut of retroviral variation.

The retroviral DNA polymerase, or reverse transcriptase, lacks a 3' exonuclease proofreading activity. This causes a high mutation rate with the result that genetic diversity and drug resistance are increased. Only combination chemotherapy has the potential to out-manoeuvre these viruses. However, there is no evidence that ongoing variation allows escape from immune surveillance, so contributing to pathogenesis.