Distribution of old and new Alu and L1 elements in genomic isochores within the human genome. Large genomic tracts of DNA can be divided into specific isochores based on their G+C content. These isochores have been shown to have various properties in terms of chromosome structure and integration of mobile elements. We have combined data from old and new Alu elements presented by Lander et al. (2001) into a single figure. This illustrates that the younger elements appear to show a similar genomic distribution relative to isochores, while the older elements have changed in frequency in opposite directions (L1, dashed arrow; Alu, solid arrow). This suggests that the two types of elements have different selective pressures in the different genomic regions. Reprinted with modification with permission from Nature 409:860–921 ©2001, Macmillan Magazines.