Genetics is one of the oldest scientific endeavors of humans. People have selectively bred various plants and animals since the Stone Age. But it was only in the mid 1940s, long after the work of Mendel (I860), the discovery of nucleic acids by Miescher (1869), and many other important findings, such as the search for transforming factors (Griffith, 1928; Avery, 1944) and chemical analysis of DNA (Chargaff in the 1940s), that it was realized that genetic information is transferred via DNA and not by, say, genetic proteins or complex polysaccharides. The double-helical structure of DNA and the mechanism of its replication were explained by Watson and Crick in 1953. In the decades that followed, the understanding of nucleic acids, their manipulation, as well as the structure and function of genes and the origin of diseases on the molecular level was rapidly improving, resulting in many new applications. One of them is gene therapy. Its aim is to improve or correct the conditions of the sickness on the molecular level. This would not only alleviate the symptoms but also eliminate the cause of the disease as well.