The same genes in different people can produce different metabolisms, and this is largely due to epigenetics: the way in which genes express themselves to form proteins. Studies of epigenetic differences are gathering pace: "One of the most ambitious large-scale projects in Human Genetics has been launched today: Epitwin will capture the subtle epigenetic signatures that mark the differences between 5,000 twins on a scale and depth never before attempted, providing key therapeutic targets for the development of drug treatments. ... Epigenetics [explores] how the actions of genes can be temporarily modified by chemical reactions that may occur either at random or by lifestyle or diet. This effect may last several generations. The plan is to look at the methylation patterns of 20 million sites (called CpG islands) in the DNA of each twin and compare them with the patterns in the co-twin. Rather than looking at similarities as in previous studies, the team will be looking for differences that explain why many identical twins don't develop the same diseases. Initially the team will focus on obesity, diabetes, allergies, heart disease, osteoporosis and longevity, but the method can be applied to every common trait or disease. ... So far this type of study has only been attempted on a handful of twins, so we want to scale it up - one thousand fold."