ABSTRACT
Many of the most serious diseases are caused by gene defects. A very important example is cancer, having its origin in a cascade of acquired gene defects (1,2). Others are the result of inherited, mostly single-gene defects, and a prominent example is cystic fibrosis (3). As first suggested in the 1970s by Friedmann and Roblin (4), it should be possible to correct the inherited or acquired gene defects by replacing the defective gene and/or by overcoming the malfunction by introducing a correct gene. Today-more than 30 years later-gene therapy, the use of therapeutic genes as drugs, is a very promising approach. During the last one and a half decades, more than 1,000 gene therapy studies have been performed (5). But at this point, gene therapy is still far from being a standard therapy.
