Reverse transcriptase is a multifunctional enzyme with 3 distinct enzymatic activities: an RNA-dependent DNA polymerase, a hybrid-dependent exoribonuclease (RNase H), and a DNA-dependent DNA polymerase. In vivo, the combination of these 3 activities allows transcription of the single-stranded RNA genome into double-stranded DNA for retroviral infection. For reverse transcription in vitro, the first two activities are utilized to produce single-stranded cDNA (Figure 1). A description of these activities is given on the next page.