[refurb]

You cant but you can place a very high probability on it.

With genetic diseases often the difference between healthy and not is a single enzyme is missing.

If you can prove the enzyme level is restored and all the downstream effects match healthy individuals, youve basically restored them to what healthy people are.

Durability of the change is the big issue. Some gene therapies wane over time.

And of course if damage had occurred before the gene therapy, that may never go away.