if one looked at your arm they could magnify it all the way down to the dna structure thats madde up of genes(revealing your entire gene blueprint). Sure they could make some gene alterations at that location,but that will only effect that location unless that alteration is a virus and then you now naturally reproduce this alteration on your own.
Are viruses the preffered method of gene alteration since they naturally invade, because it would have to take a supercomputer to alter each gene manually