Finding gene activity differences in identical twins, Ars Technica
"They're called identical twins because their genomes are identical. But even though all of their DNA is the same, they clearly are not. The environment must play a role in how identical twins and everyone else uses their genes to become who they are."
"A multitude of human research investigations currently are underway and are scheduled for upcoming expeditions aboard the space station by NASA and its international partners. The opportunity to compare the effects of spaceflight accumulated over one year and observe changes in the genetic makeup between twin brothers is new. These investigations could have lasting implications for protecting astronauts on deep space exploration missions, including travel to asteroids and Mars."
Keith's note: NASA has spent a lot of time hyping the whole twins thing. It is important to note that "identical" twins are not identical. Just take that face-to-face photo of the Kelly twins (larger view), flip one image, superimpose it on the other, align for facial features and look what you get: altered skull shape (ergo brain shape) and ear shape and placement. But they are identical - how did that happen? Hmm. Prenatal environment is likely the prime factor but other things during their early lives could be at work as well. To be certain the Kelly twins are vastly more alike at the genetic level than this crude photo comparison might suggest. But they are not identical - and they become less identical with every passing day.