Mobile pieces of DNA - transposable elements - that insert themselves into chromosomes and accumulate during evolution make up more than 40% of the human genome. Element of four types - long interspersed elements (LINEs), short interspersed elements (SINEs), LTR transposons and DNA transposons - are inserted more or less randomly throughout the human genome. These elements are conspicuously rare at the four homeobox gene clusters, HoxA, HoxB, HoxC, and HoxD, as illustrated for HoxD in Each Hox cluster is about 100 kb in length and contains 9 to 11 genes, whose differential expression along the anteroposterior axis of the 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?