There is a limited subset of things for which gene therapies can work in adults. For example, thalassemia is caused by continued abnormal production of hemoglob

There is a limited subset of things for which gene therapies can work in adults. For example, thalassemia is caused by continued abnormal production of hemoglobin. If you fix the one defective gene, future hemoglobin will now be the correct variety. However, the old hemoglobin does not magically correct itself, and it will take time to clear. Meanwhile the damage it may have done to the liver and spleen will persist. Gene therapy can also fix other conditions. For example, many complications of cystic fibrosis are caused by abnormal cilia not clearing mucus properly. You can switch the gene on to restore cilia function and clear the mucus. Sounds good so far, right? Well, not quite. Cystic fibrosis can also cause men to be born without vas deferens -- the tubes that connect the testicles to the urethra and allow sperm to leave the penis. This happens because normal male development requires working cilia at the time the vas deferens is forming -- sometime in the first trimester of pregnancy. Once you pass this point, restoring normal cilia function will not make the organs that failed to form magically spring into being. That was a developmental event that does not happen again. There are no second chances for many stages of development. We have a long way to go before we can successfully treat genetic disease before the embryo starts to form.