Structure and activities of ERV families. (a) Numbers of bases within nine ERV families present in the C57BL/6J reference genome. (b) Proportions of C57BL/6J bases belonging to each ERV TEV family relative to apparently fixed ERV sequence. These proportions reflect the variable ages and historical activities of ERV families. (c) Percentages of TEV ERVs predicted as having a proviral structure within the C57BL/6J or C57BL/6NJ lineages projected onto the phylogenetic tree from Figure 1. An older ERV insertion shows a greater tendency to be a solo-LTR than a canonical proviral form. (d) Proportions of proviral and solo-LTR structures for each TEV ERV family. Solo-LTR fractions are approximately proportional to the age of the TEVs with the exception of ETn and IAP, which have a higher percentage of solo-LTRs for their age relative to the other families.