Identical twins have long been researchers' favourite guinea pigs because they hold the key to answering one of the greatest scientific questions of our time: nature, or nurture? World-renowned genetic epidemiologist Dr. Tim Spector recently found out that by probing twins' differences, rather than their similarities, we can learn how the environment and heredity interact to actually change our genes! Research on epigenetics could help solve medical mysteries and lead to new treatments for all.