The medical mystery of what causes some twins to be born identical may have been solved by scientists in the Netherlands, raising hopes for treatment of congenital disorders that disproportionately afflict them. Identical twins form after a fertilised egg, called a zygote, splits into two embryos sharing exactly the same genes. The reason for the split is unknown. The prevailing theory has been that the biological process that leads to “monozygotic twinning” is random. But international researchers led by the Vrije Universiteit Amsterdam believe they have found a common “signature” on the DNA of identical twins. The research, published in the journal Nature Communications, looked at the epigenetic modifications in twins’ DNA, factors that can switch genes “on” or “off” without changing their underlying sequence. It found that identical twins from across the world shared similar marks at 834 points across their genome, the sum total of an organism’s DNA. The shared marks, the scientists say, can allow researchers to determine, with up to 80% accuracy whether an individual is an identical twin, including those unaware they had lost their twin in the womb, known as vanishing twin syndrome, or been separated at birth. It is yet to be proved that the chemical marks on the DNA are the cause of identical twins being conceived, as it remains possible that is a consequence of the twinning process. But it is said to be a plausible working theory, and the discovery of the common marks could be helpful for a wide group of people. As many as 12% of human pregnancies may start as multiple embryos but just under 2% carry to term, with many people unaware that they had had a twin. Nancy Segal, a developmental psychologist at California State University, told the Science News publication that it was “a very, very important finding”, as identical twins are predisposed for a variety of conditions, including spina bifida. The research raises the prospect that people with such a disorder could be one of an identical set of twins who had lost their sibling in the womb or been separated at birth.