Oncoviruses are the cause of some forms of human cancers. Dulbecco's study gave a basis for a precise understanding of the molecular mechanisms by which they propagate, thus allowing humans to better fight them. Furthermore, the mechanisms of carcinogenesis mediated by oncoviruses closely resemble the process by which normal cells degenerate into cancer cells. Dulbecco's discoveries allowed humans to better understand and fight cancer. In addition, it is well known that in the 1980s and 1990s, an understanding of reverse transcriptase and of the origins, nature, and properties of human immunodeficiency virus (HIV, of which there are two well-understood serotypes, HIV-1, and the less-common and less virulent HIV-2), the virus which, if unchecked, ultimately causes acquired immunodeficiency syndrome (AIDS), led to the development of the first group of drugs that could be considered successful against the virus, the reverse transcriptase inhibitors, of which zidovudine is a well-known Example. These drugs are still used today as one part of the highly-active antiretroviral therapy drug cocktail that is in contemporary use.