Transposable elements make up more than 40% of the human genome and are inserted more-or-less randomly throughout the genome. These elements are rare at the four homeobox gene clusters (HoxA, B, C, and D) along with an equivalent region of chromosome 22, which lacks a Hox cluster. Each cluster is about 100 kb in length and contains 9-11 genes. Why would transposable elements be so rare then in these clusters?