Scientists are just beginning to understand the effect lifestyle choices and other environmental factors have on altering gene behavior, a rapidly emerging field called epigenetics. Your life story depends upon a combination of the DNA you're stuck with plus your environment, including all the little choices and events that happen over that lifetime. But in recent years, researchers have discovered that, while DNA lays out the options, many of those life experiences - the foods you eat, the stresses you endure, the toxins you're exposed to - physically affect the DNA and tell it more precisely what to do. The cause: a kind of secondary code carried along with the DNA. Called the epigenome," this code is a set of chemical marks, attached to genes, that act like DNA referees. They turn off some genes and let others do their thing. And although the epigenome is pretty stable, it can change - meaning lifestyle choices such as diet and drug use could have lasting effects on how the body works. "The thing I love about epigenetics is that you have the potential to alter your destiny," says Randy Jirtle, who studies epigenetics at Duke University Medical Center in Durham, N.C. Jirtle compares the system to a computer: The DNA is the hardware - set and unchanging - and the epigenome is the software that tells it when, where and how to work.