Any of the deoxyribonucleic acid (DNA) polymerases present in particles of retroviruses, that is, in virions of ribonucleic acid (RNA) tumor viruses and related viruses. These DNA polymerases are coded by the retroviruses and are able to carry out DNA synthesis using an RNA template. This reaction is called reverse transcription since it is the opposite of the usual transcription reaction, which involves RNA synthesis using a DNA template. Thus, the viral DNA polymerases that carry out this reverse reaction have been called reverse transcriptases, and the viruses with this enzyme are called retroviruses because they reverse the usual flow of genetic information. Such viruses are much discussed now, since the etiologic agent of acquired immune deficiency syndrome (AIDS) is a retrovirus.