Baltimore extended this work and examined two RNA tumor viruses, Rauscher murine leukemia virus and Rous sarcoma virus. He went on to discover reverse transcriptase (RTase or RT) – the enzyme that polymerizes DNA from an RNA template. In doing so, he discovered a distinct class of viruses, later called retroviruses, that use an RNA template to catalyze synthesis of viral DNA. This overturned the simplified version of the central dogma of molecular biology that stated that genetic information flows unidirectionally from DNA to RNA to proteins. Reverse transcriptase is essential for the reproduction of retroviruses, allowing such viruses to turn viral RNA strands into viral DNA strands. The viruses that fall into this category include HIV.