An enzyme, occurring in retroviruses, that catalyses the formation of double-stranded DNA using the single RNA strand of the viral genome as template. This enables the viral genome to be inserted into the host’s DNA and replicated by the host. Reverse transcriptase is thus an RNA-directed DNA polymerase. The enzyme is used in genetic engineering for producing complementary DNA from messenger RNA, e.g. in determining levels of gene expression in the technique called RT-PCR (see polymerase chain reaction). See also RNA sequencing.