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Retrotransposon repeats account for just over ___1___ % of our genome and are classified into three broad families. One family resembles a class of RNA virus, known as a ___2____. Like a ___2____, they contain direct repeats at their ends, known as ___3____ ____4____ repeats, and full-length family members have the same gene structure as a simple ____2____. A second family of retrotransposon repeats, known as ____5____ , has some full-length copies with sizes of 6-8 kb, and like the retrovirus-like family some of them are able to make a specialized DNA polymerase known as _____6____ ____7_____. A third family of retrotransposon repeats, known as ___8____, have short full-length sequences of between 100 and 300 bp, and are exemplified by ___9____ repeats, the most prolific DNA sequence in the human genome, with a copy number of more than ____10____ ____11_____ repeats. Only a small fraction of the retrotransposon repeats can actively transpose (most are truncated copies or have ____11_____ mutations). ___8____ repeats and other _____8_____ are unable to make a ____6____ ____7_____ but very occasionally do transpose using a ____6____ ____7_____ produced by another retrotransposon.