HIV reverse transcriptase. Computer model of the enzyme reverse transcriptase (RT), from the human immunodeficiency virus (HIV). This virus causes AIDS (acquired immune deficiency syndrome). The colour-coding relates to the amino acids which make up the enzyme, with each shown as a different colour. Cylinders represent helical regions in the enzyme, and the flat arrows represent sheet regions. RT is used by HIV in infected cells to build double-stranded DNA (deoxyribonucleic acid) molecules from the virus' own single-stranded RNA (ribonucleic acid). In this way HIV instructs the infected cell to form more viruses.