This concise guidebook consolidates the main concepts of the cancer gene theory, and provides a framework for understanding the genetic basis of cancer. Focused on the most highly representative genes that underlie the most common cancers, the book is aimed at advanced undergraduates who have completed introductory courses in genetics, biology and biochemistry, medical students, and house medical house staff preparing for board examinations. Primary attention is devoted to the origins of cancer genes and the application of evolutionary theory to explain why the cell clones that harbor cancer genes tend to expand.