Renato Dulbecco (1914-2012) was an Italian-born American virologist who won the 1975 Nobel Prize in Physiology or Medicine for his work on reverse transcriptase, which viruses use to translate their RNA into DNA inside the cells they infect. Dulbecco and his group demonstrated that the infection of normal cells with certain types of viruses (oncoviruses) led to the incorporation of virus-derived genes into the host-cell genome, and that this event lead to the transformation (the acquisition of a tumor phenotype) of those cells. As demonstrated by Temin and Baltimore, who shared the Nobel Prize with Dulbecco, the transfer of viral genes to the cell is mediated by an enzyme called reverse transcriptase (or, more precisely, RNA-dependent DNA polymerase), which replicates the viral genome (in this case made of RNA) into DNA, which is later incorporated in the host genome. Dulbecco was actively involved in research into identification and characterization of mammary gland cancer stem cells until December 2011. He lived to be 97 and died of natural causes.