All replication-competent retroviruses possess a characteristic enzyme, reverse transcriptase (RT), which is present at 20–70 mol/virus particle (1 –3 ). The enzyme is cleaved, and thereby activated, from an inactive precursor by the action of another retroviral enzyme, the viral protease. All RTs possess three distinct enzymatic activities: (1) an RNA-dependent DNA polymerase, which is the RT in the strict sense of the word, (2) an RNase H, and (3) a DNA-dependent DNA polymerase. After infection of a new host cell, these different activities serve in turn to synthesize a cDNA of the viral RNA, to degrade RNA from the cDNA-RNA heteroduplex, and to duplicate the cDNA strand (reviewed in ref. 4 ).