The study of the diseases collectively labeled cancer has involved a diverse set of approaches, ranging from clinical evaluation and treatment to tumor induction in experimental organisms, and, perhaps most significantly, into the molecular genetics of cell proliferation and growth control. This book is organized around the remarkable body of knowledge which has developed in recent years from studies of the genes involved in cancer and cell regulation. There is now a diverse collection of genes known which, when inappropriately expressed, produce dramatic effects on cells and in animals. A considerable number are directly implicated in various naturally occurring cancers. Many of these oncogenes have been initially identified through their presence in tumor viruses, employing assays which have demonstrated their capabilities to transform cells and thereby subvert processes which regulate normal cellular activities.