How can defective genes be turned off?

We put this question to Adrian Thrasher from Great Ormond Street Hospital:

Well, usually we don't need to turn off a defective gene because the nature of the mistake is usually such that the gene is off anyway. So if we put a functioning gene back in, that fixes the problem. In some cases it is necessary to turn off the defective gene and there are several strategies that can address it. So for example we can address it at the RNA level, so encourage RNA degradation by using homologous RNA sequences; or, in fact, you could target enzymes that will make deletions within that specific gene and thereby destroy it.