Howard Temin participated in two Lindau Meetings and gave lectures at both of them. The first lecture, in 1981, is rather technical and the present lecture is more easily understood. It is an unusually clear presentation of the way that certain so-called retroviruses act and an explanation of how they can cause cancer. Central to the understanding is the process of reverse transcription, which was discovered independently by Temin and David Baltimore around 1970. With transcription is meant the process when the double stranded DNA makes a single stranded RNA copy, which then can be used as a messenger to the protein factory of the cell, ordering it what kind of proteins to produce. Some viruses only contain RNA, but can anyway modify the DNA of a cell into which the virus has entered to transform it into a cancer cell. How this can be done was a mystery until Temin and Baltimore, using methods developed by Temin’s teacher Renato Dulbecco, made their discovery of reverse transcription. In his lecture, Temin goes one step further in the understanding of how retroviruses act and mainly talks about oncogenes, proto-oncogenes and viral oncogens. An oncogene is a gene which causes a cell to become a cancer cell and which, e.g., may have been introduced into the cell by a virus. As shown by two other Nobel Laureates, Michael Bishop and Harold Varmus, the virus first picks up the gene that eventually will become an oncogene in a normal cell. The gene that is picked up is called a proto-oncogene. Temin, in his lecture, explains how this proto-oncogene first is modified by the virus to become a viral oncogene. When this viral oncogene has been introduced into the DNA of a cell, it becomes an oncogene and the cell becomes a cancer cell. It turns out that this has to do with the process of regulation. A proto-oncogene in the DNA of its original normal cell is regulated by other genes of the cell. When the virus picks up the proto-oncogene, it doesn’t pick up the regulator genes. This means that the regulation changes, so that the proto-oncogene becomes a viral oncogene. According to Temin, the number of proto-oncogenes is rather small and certainly below 40. Different viruses pick up different proto-oncogenes, causing different kinds of cancer. All this is of great interest for basic cancer research, but as Temin points out, the majority of cancers in human beings are not caused by virus infections. Instead other factors, such as environmental pollution or smoking, are important and a large proportion of human cancers are lung cancers caused by smoking. Howard Temin was not a smoker, but he acquired lung cancer anyway and passed away at age 59. Anders Bárány