John Michael Bishop (1936 - present) received one half of the Nobel Prize for Physiology or Medicine in 1989 for his discovery of the cellular origin of retroviral oncogenes – genes that code for proteins that increase the malignancy of tumor cells. Working with fellow Nobel Laureate Harold E. Varmus, Bishop showed that healthy cells contain dormant viral oncogenes that, when triggered, cause cancer. They found that healthy chicken cells contain a gene similar to a cancer-causing gene in the Rous sarcoma virus and concluded that the virus had taken up the gene responsible for cancer from normal cells. (After the virus infected the normal cells and replicated, it incorporated the gene into its own genetic material.) By 1989, scientists had identified more than 40 genes having cancer-causing potential in animals.