Analysis of fixed versus polymorphic retrotransposon insertions. (A) Analysis of average GC content flanking polymorphic (P, white) and fixed (F, gray) retrotransposons. GC content was calculated in 5 kb flanking the repeat. The number of repeat instances is also indicated. GC content is significantly higher for regions flanking fixed compared to polymorphic Alus ; the same holds for SVAs. (B) Analysis of polymorphic (white) and fixed (gray) retrotransposon relative frequency in different isochores (L1 to H3, ordered from 1 to 5, as described in ). Fixed Alus are significantly enriched in heavy isochores compared to polymorphic instances.