David Baltimore (1938- present) was awarded one-third of the 1975 Nobel Prize in Physiology or Medicine for showing how cancer viruses alter the genetic material of their host cells. Baltimore studied how certain tumor-causing RNA viruses called retroviruses replicate after they infect a cell and showed that they contain an enzyme called reverse transcriptase that transcribes single-stranded RNA into double-stranded DNA. The newly formed viral DNA then integrates into the infected host cell, which can transform it into a cancer cell. Baltimore also studied vesicular stomatitis virus (VSV), an RNA virus from the same family as the Rabies virus, and showed that it reproduces by an RNA polymerase but does not involve DNA. The discovery of reverse transcriptase has become important in recombinant DNA technology â€“ in which an artificial DNA sequence is created by combining other DNA sequences -- and has helped scientists understand the role of viruses in the development of cancer.