One of the ways that we can understand that genetics is a factor is by looking at something called the concordance rate. In other words, if one sibling has autism, what's the probability that another sibling in that family will have autism? And we can look in particular at three types of siblings: identical twins, twins that actually share 100 percent of their genetic information and shared the same intrauterine environment, versus fraternal twins, twins that actually share 50 percent of their genetic information, versus regular siblings, brother-sister, sister-sister, also sharing 50 percent of their genetic information, yet not sharing the same intrauterine environment. And when you look at those concordance ratios, one of the striking things that you will see is that in identical twins, that concordance rate is 77 percent. Remarkably, though, it's not 100 percent. It is not that genes account for all of the risk for autism, but yet they account for a lot of that risk, because when you look at fraternal twins, that concordance rate is only 31 percent. On the other hand, there is a difference between those fraternal twins and the siblings, suggesting that there are common exposures for those fraternal twins that may not be shared as commonly with siblings alone.