Identical twins possess exactly the same set of genes. Yet as they grow older, they may begin to display subtle differences. They may start to look different, develop different diseases or slide into different persons. 1. ______. Women who are identical twins may differ with their fertility or 2. ______. the age at which they reach menopause. These discrepancies usually attributed to ill-defined differences in environment. 3. ______. But a whole new level of explanation has been opened up by a generic survey 4. ______. showing that identical twins, because they grow older, differ increasingly in 5. ______. what is known as their epigenome. The term refers to natural chemical modifications that occur in a person’s genome shortly before 6. ______. conception and that act on a gene as a gas pedal or a brake, marking it for higher or lower activity. 7. ______. Identical twins have the same set of epigenetic mark on the genome 8. ______. when they are born. But differences in the epigenome disappear 9. ______. as the twins grow older and become greater the longer they live together, 10. ______. say a team of researchers led by Dr. Manel Esteller of the Spanish National Cancer Center in Madrid.