It's a promising new approach, says Arturas Petronis, who heads the epigenetics lab at Toronto's Centre for Addiction and Mental Health. Researchers have known for some time that complex disorders such as autism are highly heritable. But intensive scrutiny of DNA sequences themselves hasn't revealed why twins like Sam and John diverge so much in their behavior. "After 30 years of molecular genetic studies we can explain only about 2 or 3 percent of inherited predisposition to psychiatric disease," he says. The rest is still a mystery. That's one reason the National Institutes of Health created the Roadmap Epigenomics Program in 2008, providing $185 million for research into epigenetics at more than 40 labs.