Cohen, Boyer and associates developed the first practical method for cloning genes, by the formation of recombinant plasmids which can be used to infect plasmid-free bacteria. The authors 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 paper is available from the PNAS at this link.

The method that Cohen and Boyer invented eventually resulted in U.S. patent No. 4,237,224 granted to Cohen and Boyer on December 2, 1980. This patent, "Process for producing biologically functional molecular chimeras," was the foundation of the biotechnology industry. 

Cohen published a semi-popular account of this research as "The manipulation of genes," Scientific American, 233 (1975) 24-33.