Great changes in science come about in different ways. Sometimes, they are the result of new concepts that transform our way of looking at things, or give us new insights into areas of knowledge which had previously been obscure. Such a revolution in biology had already occurred in the two decades before Cohen’s paper, with the realization that the fundamental stuff of inheritance is DNA, with the discovery of DNA’s remarkable structure, and with the unscrambling of the genetic code. Other dramatic changes in science have been more technical than conceptual, and are no less important for that. Cohen’s paper describes the first methods for manipulating DNA in ways that began to give the experimenters a measure of control over these molecules, hence enabling manipulation of the genetic properties of the organisms that contain them. Humans have, of course, been selectively breeding organisms for particular traits for millennia: domestication of wild plants for crops was a hugely successful early experiment in genetic engineering. But with the advent of what are commonly called recombinant DNA techniques, the degree to which we can produce predetermined genetic changes with high precision has grown to the point where now it is commonplace to make bacteria or plants that produce human proteins, to tinker with the basic structures of enzymes to

alter their activity or stability, or to pull a single gene from the tens of thousands present in a human chromosome and identify within in it a single changed base that may give rise to a crippling genetic disease.