This episode covers the 1975 Nobel Prize in Physiology or Medicine, which was awarded to David Baltimore, Renato Dulbecco, and Howard Temin. The Nobel Assembly at the Karolinska Institute chose to give Baltimore, Dulbecco, and Temin the award “for their discoveries concerning the interaction between tumor viruses and the genetic material of the cell”. Topics include the identification of Rous Sarcoma Virus as the first known retrovirus, the discovery of an enzyme called reverse transcriptase that can synthesize a DNA copy of a molecule of RNA, and how reverse transcriptase has become an essential tool for solving some of biology’s biggest problems.