Although identical twins may look the same to the human eye, new research has discovered that identical twins may in fact not be 100% genetically identical. Writing in the American Journal of Human Genetics, an international team of US and European researchers found that when it comes to the genetics of identical twins there are in fact tiny differences and that they are relatively common. Until now, researchers have largely assumed that identical twins are genetically identical, reasoning that any variations between them were solely due to environmental factors. So, for instance, the fingerprints of identical twins differ because they each experience slightly different conditions while in the womb. In this new study, researchers analysed 19 pairs of identical twins. Although they did possess nearly identical genomes, closer study revealed differences in the copy number variation (CNV) in DNA. CNV occurs when a set of coding letters in DNA are missing, or when extra copies of segments of DNA are produced. Such variations could explain why one identical twin can suffer from a disorder while the other remains healthy. 'The presumption has always been that identical twins are identical down to their DNA,' explain the study's lead authors, Carl Bruder and Jan Dumanski, both from the University of Alabama's Department of Genetics. 'That's mostly true, but our findings suggest that there are small, subtle differences due to CNV. Those differences may point the way to better understanding of genetic diseases when we study so-called discordant monozygotic twins....a pair of twins where one twin has a disorder and the other does not,' they added. Drs Bruder and Dumanski think their findings indicate that CNV may play a critical role in one twin developing a particular disease, such as Parkinson's, while the other does not. This can be efficiently studied in identical twins. 'Changes in CNV may tell us if a missing gene, or multiple copies of a gene, are implicated in the onset of disease,' Dr Bruder said. 'If twin A develops Parkinson's and twin B does not, the region of their genome where they show differences is a target for further investigation to discover the basic genetic underpinnings of the disease.' The research was carried out by scientists at the University of Alabama; USA, Leiden University Medical Centre and VU University, the Netherlands; and Uppsala University and the Karolinska Institute, Sweden.