A Reverse transcriptase (RT) is an enzyme used to generate complementary DNA (cDNA) from an RNA template, a process termed reverse transcription. It is mainly associated with retroviruses. It should be noted however that also non-retroviruses use RT (for example, the hepatitis B virus, a member of the Hepadnaviridae, which are DsDNA-RT viruses, while retroviruses are ssRNA viruses). RT inhibitors are widely used as antiretroviral drugs. RT activities are also associated with the replication of chromosome ends (telomerase) and some mobile genetic elements (retrotransposons). Retroviral RT has three sequential biochemical activities: (a) RNA-dependent DNA polymerase activity, (b) ribonuclease H, and (c) DNA-dependent DNA polymerase activity. These activities are used by the retrovirus to convert single-stranded genomic RNA into double-stranded cDNA which can integrate into the host genome, potentially generating a long-term infection that can be very difficult to eradicate. The same sequence of reactions is widely used in the laboratory to convert RNA to DNA for use in molecular cloning, RNA sequencing, polymerase chain reaction (PCR), or genome analysis. Well studied reverse transcriptases include: HIV-1 reverse transcriptase from human immunodeficiency virus type 1 () has two subunits, which have respective molecular weights of 66 and 51 kilodaltons. M-MLV reverse transcriptase from the Moloney murine leukemia virus is a single 75 kDa monomer. AMV reverse transcriptase from the avian myeloblastosis virus like HIV also has two subunits, a 63kDa subunit and a 95kDa subunit. Telomerase reverse transcriptase that maintains the telomeres of eukaryotic chromosomes