The burgeoning new science of epigenetics offers a cornucopia of insights—some comforting, some frightening. For example, the male fetus may be especially vulnerable to certain common chemicals in our environment, in ways that damage not only his own sperm but also the sperm of his sons. And it's epigenetics that causes identical twins to vary widely in their susceptibility to dementia and cancer. But here's the good news: unlike mutations, epigenetic effects are reversible. Indeed, epigenetic engineering is the future of medicine.