From the website of RettSyndrome.org
"Gene modifying therapies as a whole offer great promise as potentially curative treatments for genetic disorders like Rett syndrome because they target such diseases at their root cause: genetic mutations.
"The most well studied gene modifying therapies rely on an approach called gene replacement. In gene replacement therapy, a working copy of a gene of interest is added to cells, effectively replacing the mutated disease-causing copy of the gene.
"However it is almost always the protein product of the gene (not the gene itself) that triggers the dysfunction associated with the disease. Gene replacement addresses this issue because a healthy copy of the gene is inserted and makes a normal RNA, which in turn is used to create a working protein...."
Read more here.