How does gene therapy help cystic fibrosis?

Not available yet: Gene therapy is not available for cystic fibrosis (CF) yet (2012). Having said that, the underlying concept is that since CF is a genetic disease (meaning there is a defect in a gene), by repairing that defect through gene therapy, one can, hopefully, cure the disease.

Not effective yet: As of yet, there are no effective gene therapies for cystic fibrosis. However, there is a new drug in clinical trials right now that might do just this. The gene therapy works, in general, by "fixing" the underlying genetic defect that causes cystic fibrosis. The cftr protein is at the core of this disease etiology, and gene therapy helps the cftr protein form and operate normally.