At first glance identical twins seem, well, identical. In fact many of these sibling pairs show minor physical variations and differences in characteristics such as susceptibility to disease. Just what causes these dissimilarities is unclear. But a new report further suggests that epigenetic factors--that is, differences in how the genome is expressed--could be responsible.

Mario F. Fraga of the Spanish National Cancer Center and his colleagues studied 160 monozygous twins ranging from three to 74 years of age. They analyzed two epigenetic phenomena along the entire genome and compared the results for each set of twins. The processes, DNA methylation and histone acetylation, both govern gene expression and can amplify or dampen the effects of particular genes. The team determined that early in life, twins were indistinguishable in the manner in which their genes were expressed. Among older sets of twins, however, significant differences in the gene-expression portraits were apparent for 35 percent of the study group. (The image above shows methylation patterns for three-year-old twins (left) and 50-year-old twins (right), with the differences highlighted in red.) In addition, twins who had spent the most time apart and had more divergent medical histories exhibited the greatest epigenetic differences.

Environmental factors, including smoking habits, physical activity levels and diet, can influence epigenetic patterns and may help explain how the same genotype can be translated in different ways, the scientists say. They suggest that future studies should investigate specific mechanisms that cause this so-called epigenetic drift in identical twins. A report describing the work is published online this week by the Proceedings of the National Academy of Sciences.