FUNCTION AND LOCATION. “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. 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.”
http://www.nature.com/scitable/topicpage/transposons-the-jumping-genes-518