Three models have been proposed to explain retroviral recombination. Data support the minus strand exchange model which proposes that since the retroviral virion contains two essentially identical RNA molecules, an extended single-stranded DNA molecule is left tailing behind. This single-stranded DNA tail is free to form a hybrid duplex with another RNA molecule. This second RNA molecule will replace the original RNA as the template for continued synthesis. However, a few puzzles, such as negative interference, need to be resolved. To do so, a modified model is proposed that a recombination occurs only when the other strand RNA molecule also binds a reverse transcriptase which replaces the original one and continues the minus strand DNA synthesis using the other strand RNA as the template.