From an in vitro analysis of the DNA-synthesizing abilities of certain specifically mutated forms of the heterodimeric reverse transcriptase of human immunodeficiency virus type 1, we can conclude that in a heterodimer, the functionality of p66 is necessary while the functionality of the p51 subunit is not needed. Conversely, p51 is not able to catalyze DNA synthesis when associated with p66, and yet when the p66 protein is absent, p51 can function. These conclusions applied to DNA synthesis on heteropolymeric RNA and DNA templates.