||P.165 bottom paragraph: "The genomes of higher eukaryotes are crowded with DNA elements that are repeated thousands or millions times. For instance, they amount to ~40–45% of human and mouse genomes. Many of these repeats were generated through the activity of transposable elements or transposons that can insert their copies into new chromosomal locations. Transposons are divided into two classes according to whether their replication is mediated by RNA (class I) or DNA (class II). Both classes include autonomous and nonautonomous elements. Autonomous transposons have open reading frames (ORFs) encoding proteins essential for transposition, whereas nonautonomous transposons encode no proteins and rely on the replication machinery of the autonomous transposons. Integration of nearly all transposons results in duplication of a short genomic sequence at the insertion point (target site duplication)."