The first disease to be tackled by this method was
Cystic Fibrosis, a horrible genetic disorder where only a single protein is faulty. Unfortunately, this is a chloride channel pump (CFTR) which is used by the lungs to maintain the ionic balance of the
mucous membranes. Sufferers have difficulty removing the fluid that tends to build up in the lung cavity and have to have it pounded out of them while lying on their backs.
As if that wasn't enough, the chloride imbalance impairs the background defences of the lung, which prevent bacteria growing there. This means that infection can set in and life expectancy is low. Addition of the gene for CFTR is only temporary, but it presumably increases the quantity and quality of life. Something to think about if you disagree with GMOs - which is essentially what these people are.