An endonuclease is an enzyme that cuts at a specific site the DNA of the chromosome with which its gene’s own chromosome is paired. Because DNA breaks are very threatening to genome integrity, cells rush to repair them, often by using the other chromosome of a pair as a template. In doing so, they copy the gene for the endonuclease into the joint made between the two broken ends of the cut chromosome. If this repair occurs in a germ line cell, both eggs and sperm will carry the endonuclease gene together with any cargo genes that genetic engineers may have attached to it.

Because it is hard to change the natural site at which a homing endonuclease cuts DNA, Dr. Burt’s proposed gene drive systems could not easily be put into practice. All that changed three years ago with the invention of Crispr-Cas9 gene editing. The technique is based on a natural system that evolved in bacteria as a defense against invading viruses.

The bacteria store DNA samples from these invasive viruses in a DNA library, called Crispr, that is part of their genome. When a virus attacks, endonucleases such Cas9 (Cas stands for Crispr-associated) are primed by the Crispr library to cut viral DNA of the same sequence.

After recovering from their amazement that organisms as small as bacteria possessed an adaptive immune system, biologists realized that they could take over the Cas9 endonuclease and make it cut DNA at any site of their choosing by providing it with a synthetic guide sequence instead of one from the Crispr library. The use of Crispr-Cas9 for genome editing was first published in 2012 by Jennifer Doudna of the University of California, Berkeley, and Emmanuelle Charpentier, now at the Max Planck Institute for Infection Biology in Berlin. But Feng Zhang, of the Broad Institute in Cambridge, Mass., was the first to file a patent, which Berkeley lawyers are challenging.

The Crispr-Cas9 technique gives biologists unprecedented power to edit DNA. With the ability to cut DNA at a specific site, they can let the cell’s DNA repair machinery paste in new sequences, usually a gene of interest, in the process of annealing the two cut ends of the DNA molecule.

Uncertain Ecological Effect

In April, two biologists at the University of California, San Diego, Valentino M. Gantz and Ethan Bier, caused a stir with a gene drive system that carried a gene for albinism into laboratory fruit flies. Their drive was astonishingly efficient: Within two generations, some 97 percent of the fruit flies had been rendered pale by the mutation. Although gene drives may not spread so quickly in natural populations, which are more variable, the experiment demonstrated the vast potential of the method for modifying pest populations.