Will adding a normal copy of the gene fix the problem in the affected tissue? Or could getting rid of the defective gene fix it? Sometimes when a gene is defective, no functional protein is being made from it. In cases like these, adding a functional copy of the gene could correct the problem. But sometimes a defective gene codes for a protein that starts doing something it shouldn't or prevents another protein from doing its job. In order to correct the problem, you would need to get rid of the misbehaving protein.