Another way to define gene therapy is the replacement of defective genes with genes which are healthy. Once the healthy genes combine with the DNA of the patient, the DNA will trigger new cells and target what they need to target. Among the diseases that are considered "treatable" by GT are cystic fibrosis and Parkinson’s disease, cancer, Huntington’s disease, retinal degeneration, types of blindness, phenylketonuria (PKU), AIDS, ageing and many more. GT has been applied with partial success in cases of blindness. Yet, as we have talked about it as a society, not once has a case of GT been successful enough to, for instance, cure Cancer, AIDS, or reverse aging. The FDA has not approved as of yet any product that is conducive to the application of GT, and not enough practice has been given to prove that it works. It is then a matter of time to decide whether it is worth investing time and money on it.