Harold Varmus is best known for his studies into the genetics of cancer. Harold discovered that under certain circumstances, the activation of genes known as oncogenes — which usually control the normal growth and division of healthy cells — can lead to cancer. This finding enabled the development of mouse models of human cancers and permitted the in-depth study of the function of genes implicated in cancer.
Harold exploited RNA viruses known as retroviruses during his investigation of oncogenes. Thus, he is also renowned for his studies of the behaviour of these viruses, ranging from the nature and origin of their transformative genes to their potential to cause genetic changes.