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.