Answer:
The fact that roughly half of the human genome is made up of TEs, with a significant portion of them being L1 and Alu retrotransposons, raises an important question: What do all these jumping genes do, besides jump? Much of what a transposon does depends on where it lands. Landing inside a gene can result in a mutation, as was discovered when insertions of L1 into the factor VIII gene caused hemophilia (Kazazian et al., 1988). Similarly, a few years later, researchers found L1 in the APC genes in colon cancer cells but not in the APC genes in healthy cells in the same individuals. This confirms that L1 transposes in somatic cells in mammals, and that this element might play a causal role in disease development (Miki et al., 1992).