Repetitive sequences in the human genome.
About half of our DNA bears homology to known classes of repeats (left chart). The largest class of repeats is the non-LTR retrotransposons, which consists mostly of LINE-1 (L1), L2, MIRs, and Alu elements (right chart). L2 and MIR sequences are not currently active, but subsets of L1 (17.88%), Alu (10.76%), and SVA sequences (not shown, 0.1%) are currently mobile in human genomes and are sources of genetic polymorphisms. Proportions were determined using a RepeatMasker (version rm-20110920, default settings, RepBase sequence database version 16.08) analysis of the Human February 2009 (GRCh37/hg19) assembly. LTR, long terminal repeat retrotransposons; L1, long interspersed element–1; L2, long interspersed element–2; MIR, mammalian wide interspersed repeat; Alu, a short interspersed element named for the AluI restriction enzyme; SVA, a composite retrotransposon consisting of short interspersed repeat (SINE-R), variable number tandem repeat (VNTR), and Alu like sequence segments.