Cystic Fibrosis can be caused by deletion mutations or by substitution mutations. Mutations alter nucleotide order which can alter gene expression or protein shape and function. Gene therapy can involve either germ line or somatic line cells. Ethical issues are especially important in germ line gene therapy. How is CFTR made and delivered to cell membrane mutant CFTR never makes it to the cell membrane.
Different proteins found in different cells account for the differences.
While this has been done on organisms other than humans, no government has given the approval for this type of therapy with humans. If this therapy was used on humans, we would be altering not only the genes of the individual, but the genes of all of this individual s offspring. This type of intervention is interfering with the normal course of evolution. Since genes interact with each other in ways still not understood, it is unknown how the new allele will interact with the other genes of the individual. It is not fair to the child, and his or her children, to insert alleles into the genome.