The concepts of gene therapy arose initially during the 1960s and early 1970s whilst the development of genetically marked cells lines and the clarification of mechanisms of cell transformation by the papovaviruses polyoma and SV40 was in progress. With the arrival of recombinant DNA techniques, cloned genes became available and were used to demonstrate that foreign genes could indeed correct genetic defects and disease phenotypes in mammalian cells in vitro. Efficient retroviral vectors and other gene transfer methods have permitted convincing demonstrations of efficient phenotype correction in vitro and in vivo, now making gene therapy a broadly accepted approach to therapy and justifying clinically applied studies with human patients.