David Baltimore, at MIT, and Howard Temin, at the University of Wisconsin, independently and simultaneously discover in 1970 an enzyme that contradicts the so-called "central dogma" of molecular biology prevailing until then, which assumed that genetic information always flows in one direction, from DNA to RNA and from RNA to proteins. Both identify that certain viruses — retroviruses, among which HIV would be found decades later — store their genetic information as RNA, not DNA, and possess their own enzyme, reverse transcriptase, capable of performing the opposite of the usual process: copying their viral RNA into a DNA molecule that then inserts into the infected cell's genome. The discovery explains the mechanism by which certain viruses can cause cancer by inserting viral genetic material — including oncogenes — directly into the host cell's chromosomes, work that Renato Dulbecco had begun investigating in the 1950s by studying how tumor viruses transform normal cells into cancerous ones. Reverse transcriptase also becomes an indispensable technical tool of genetic engineering, allowing scientists to convert messenger RNA into complementary DNA that can be manipulated in the laboratory, and is the target enzyme of the main antiretroviral drugs developed against HIV from the 1980s onward.