Identical twins share the same genes and are often startlingly alike. Why, then, should they often meet such different fates one twin developing a serious disease like cancer while the other remains unscathed? In a compelling scientific detective story, The explores the provocative idea that there may be more to inheritance than genes alone. New clues reveal that a second epigenetic chemical code sits on top of our regular DNA and controls how our genes are expressed, turning them on or off with dramatic consequences for our health.