What fraction of the body’s cells would have to be modified to correct Duchenne’s Muscular Dystrophy or hypercholesteremia?  I don’t know, but it would be interesting to find out.

Your question brings to mind something I’ve wondered about. If a person has a disease caused by a genetic mutation, how do you correct all (or enough of) the typos?

With a mutation that occurred at conception every single cell in the body would have a bad copy. Presumably you would not have to correct every cell in the body to improve a bubble boy’s immune system, but how much is enough, and how as a practical matter do you do this? This is a question not just for exotic genetic therapy but also the crude therapy currently used. How is one corrected cell enough? And when that cell dies and is replaced by another, wouldn’t the replacement have a bad copy?

To change eye color I presume we’d only have to change some cells in the eye, not the whole body. Maybe you could administer that “cut and paste” correction with eye drops. How about blond gene shampoo?