retrovirus, type of RNA virus that, unlike other RNA viruses, reproduces by transcribing itself into DNA. An enzyme called reverse transcriptase allows a retrovirus's RNA to act as the template for this RNA-to-DNA transcription. The resultant DNA inserts itself into a cell's DNA and is reproduced along with the cell and its daughters. The life cycle is completed when the viral DNA in selected daughter cells makes an RNA copy of itself that covers itself in a protein coat and leaves the cell. Retroviruses sometimes destroy the cells whose DNA they alter, as with HIV, the virus that causes AIDS, and sometimes cause them to become cancerous, as with the viruses that cause certain leukemias. Lentiviruses are retroviruses that cause slowly progressing diseases, such as AIDS.