 In the classical period of molecular genetics, many central mechanisms pertaining to DNA replication, gene regulation, and protein synthesis were described. While DNA served as a template in its own replication, it was not self-replicating or autocatalytic; many enzymes and other molecules were required in this process, such as DNA polymerase and DNA ligase. Furthermore, around 1970, several enzymes specific for bacteria or viruses were described such as reverse transcriptase, which enabled viruses to translate RNA into DNA, and restriction enzymes, which cleaved DNA as part of a bacterial defense system. Next to explaining important cellular processes, taken together, these enzymes could be used as a toolkit to manipulate DNA in vitro, in bacterial systems and eventually also in eukaryotic cells. In 1972, Paul Berg (born 1926) and colleagues published a study demonstrating the possibility to construct DNA molecules from fragments originating from different organisms by using these enzymes as tools (Jackson et al. 1972). These molecular tools not only led to recombinant DNA technology and genetic engineering, but also facilitated the further molecular characterization of genes by isolating and amplifying them in bacteria (molecular cloning) (Morange 2020: Ch. 16).