In 1973 Stanley Cohen, Annie Chang, Robert Helling, and Herbert Boyer demonstrated that if DNA is fragmented with restriction endonucleases and combined with similarly restricted plasmid DNA, the resulting recombinant DNA molecules are biologically active and can replicate in host bacterial cells. Plasmids can thus act as vectors for the propagation of foreign cloned genes.
This was the first practical method of cloning a gene, and a breakthrough in the development of recombinant DNA technologies and genetic engineering.
Cohen, Chang, Boyer and Helling, “Construction of Biologically Functional Bacterial Plasmids in Vitro,” Proc. Nat. Acad. Sci. 70 (1973): 3240-3244