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In the past seven to eight years we have witnessed the development of a new DNA technology that has fundamentally altered our approach to modern genetics. The basic ingredients of this new technology are the cleavage-site-specific restriction enzymes: a special class of bacterial endo-nucleases that can recognize specific nucleotide sequences in duplex DNA and produce double-stranded cleavages. Using a collection of these en-zymes, each with its own particular sequence specificity, DNA molecules may be cleaved into unique sets of fragments useful for DNA sequencing, chromosome analysis, gene isolation, and construction of recombinant DNA. The latter, combined with the concept of molecular cloning, has given birth to the new field of genetic engineering, and from this are expected many new and exciting medical and research applications. My own role in these developments occurred primarily in the period of 1968-1970 when my colleagues and I made the chance discovery of the first of the cleavage-site-specific restriction enzymes. I should like to briefl