At least half the human genome consists of mobile elements, such as LINEs, some of which can jump around the genome. These elements have been crucial in genome evolution, but they may also contribute to human diversity. Barbara McClintock won the Nobel Prize in Physiology or Medicine in 1983 for predicting the existence of mobile elements, pieces of DNA that move from one place in the genome to another. McClintock called them controlling elements' and proposed that they could account for developmental differences among individuals of a species — explaining, for example, the differences in maize-kernel colour that she observed. Although her ideas were not well received at the time, they have proven to be remarkably prescient. On page 903 of this issue, Muotri et al.2 provide evidence that mammalian mobile elements may have a role in creating "the uniqueness of individuals within a population".
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