442 Chapter 13 • The Dynamic Genome: Transposable Elements the population because they are less likely to cause a mutation. Insertions into exons are said to be subjected to negative selection. Second, humans, as well as all other multicellular organisms, can survive with so much mobile DNA in the genome because the vast majority is inactive and cannot move or increase in copy number. Most transposable-element sequences in a genome are relics that have accumulated inactivating mutations over evolutionary time. Others are still capable of movement but are rendered inactive by host regulatory mecha-nisms. The epigenetic mechanisms that serve to inacti-vate transposable elements will be discussed in more de-tail later in this chapter. There are, however, a few active LINEs and Alu s that have managed to escape host con-trol and have inserted into important genes causing sev-eral human diseases. Three separate insertions of LINEs have disrupted the factor VIII gene causing hemophilia A. At least 11 Alu insertions into human genes have been shown to cause several diseases, including hemo-philia B (in the factor IX gene), neuroFbromatosis (in the NF1 gene), and breast cancer (in the BRCA2 gene). The overall frequency of spontaneous mutation due to the insertion of class 2 elements in humans is quite low, accounting for less than 0.2 percent (1 in 500) of all
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