ABOVE: Two sets of identical twins
Sets of identical twins famously have much in common with one another. In a Dutch study published September 28 in Nature Communications, scientists find that they also have something in common with other identical twins the world over: a set of matching marks on their DNA. The researchers studied around 6,000 pairs of twins of varying ages from around the world, analyzing hundreds of thousands of sites on their genomes. Their finding of 834 sites in the genome with marks distinct to identical, but not fraternal, twins could provide clues to how identical twins come about, researchers say.

While the basics of how identical twins originate is clear, the mystery lies in why. Identical twins arise from a single fertilized egg, or zygote, splitting into embryos with duplicate DNA, but what causes the split remains unknown. Zygote splitting was once thought to be completely random. The findings of the new study call that idea into question, raising the possibility that certain epigenetic changes may drive the split. Twin researcher and developmental psychologist Nancy Segal of California State University, Fullerton, tells Science Newsthat thestudy could lead to a better understanding of "what might cause a fertilized egg to split and form monozygotic [identical] twins."

Alternatively, Baylor College of Medicine epigeneticist Robert Waterland tells Science News that the epigenetic marks could be a consequence — not the cause — of zygote splitting. "They could be like a persistent molecular scar of the [splitting] process itself," he says, adding that more research is needed to find out whether this is the case.

As part of their study, the researchers also devised a test using the marks they'd identified, and report that it was 80 percent accurate in determining whether a given person is an identical twin. The Guardian reports that such a test might be used to reveal whether someone was separated from an identical twin early in life or experienced "vanishing twin syndrome," which occurs when one twin is lost during pregnancy.

The work could also lend insight into certain disorders involving epigenetic changes, the Dutch research team writes in the paper. For example, Beckwith-Wiedemann syndrome and spina bifida disproportionately affect twins. Twin researcher Jeffrey Craig tells Science that the research could be useful in testing the idea that certain diseases are unique to identical twins, which he says would be "big news if proved true." Segal also expresses optimism about the applicability of epigenetics in twin studies, telling Science News, "This is a very, very important finding that opens up a lot of avenues of inquiry."