Mobile pieces of DNA—transposable elements—
that insert themselves into chromosomes and accumulate
during evolution make up more than 40% of the human
genome. Transposable elements of four types—long inter-
spersed nuclear elements (LINEs), short interspersed
nuclear elements (SINEs), long terminal repeat (LTR)
retrotransposons, and DNA transposons—are inserted
more-or-less randomly throughout the human genome.
These elements are conspicuously rare at the four homeo-
box gene clusters, HoxA, HoxB, HoxC, and HoxD, as illus-
trated for HoxD in Figure Q4–4, along with an equivalent
region of chromosome 22, which lacks a Hox cluster. Each
Hox cluster is about 100 kb in length and contains 9 to 11
genes, whose differential expression along the anteropos-
terior axis of the developing embryo establishes the basic
body plan for humans (and for other animals). Why do you
suppose that transposable elements are so rare in the Hox
clusters?
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