Grouping of RTE loci. A schematic overview of the procedure used to group RTE loci within each family. A hypothetical genome is shown on top with a single protein-coding gene (exons denoted by boxes). RTE loci are shown as vertical lines, with age indicated by increasing colour darkness. The RTE loci are first divided in three equally sized groups (rows below genome) based on age, then divided according to their genomic location (columns below genome). The border between proximal and distal loci is set to the median of the distances between all non-genic loci and the nearest gene. Within each age group, the location groups can now be directly compared against each other. Note that the numbers of RTEs in age groups are equal by definition, whereas this may not be the case in the location groups.