Stanley Cohen, Annie Chang, Robert Helling and Herbert Boyer show that extrachromosomal bits of DNA called plasmids act as vectors for maintaining cloned genes in bacteria. They show that if DNA is broken into fragments and combined with plasmid DNA, such recombinant DNA molecules will reproduce if inserted into bacterial cells. The discovery is a major breakthrough for genetic engineering, allowing for such advances as gene cloning and the modification of genes.
Cohen, S. N., A. C. Y. Chang, H. W. Boyer, and R. B. Helling. 1973. Construction of biologically functional bacterial plasmids in vitro. Proc. Natl. Acad. Sci. USA 70: 3240-3244. In Microbiology: A Centenary Perspective, edited by Wolfgang K. Joklik, ASM Press. 1999, p.178 [pdf]
I'll Have the Chopped Liver Please, or How I Learned To Love the Clone, ASM News 67,2001. p.555 [pdf]