A new study challenges our assumptions about mobile genetic elements by showing that one type is active in the development of the adult brain, and might therefore contribute to differences in brain function among individuals.

LINE1 (or L1) elements have had a significant hand in shaping the evolution of our genome. It would seem counterintuitive for L1s to jump about in somatic cells, as they might harm the host and halt their own propagation. But this is exactly what was seen when an engineered human L1 transgene was introduced into cultured rat neural progenitor cells or transgenic mice. Not only did the transgenes create more copies of themselves, but the L1 DNA inserted itself into the genome, affecting the expression of several neuronal genes.

These results are startling for two reasons. First, L1 elements are thought to insert themselves at random; this was clearly not the case here, where the elements preferred to jump into genes or close to them. Second, the insertion of new L1 copies disrupted gene expression and influenced neuronal differentiation. As the authors cautiously speculate, this burst of activity and its consequent effect on brain development could contribute to making individuals unique.

Whether this hypothesis is correct will depend on the answers to several questions, including how often a human endogenous L1 jumps and whether the level of retrotransposition in neural progenitor cells is, in fact, sufficient to account for changes in brain activity. One way or another, L1s are keeping us thinking.