Still, concerns are arising anew because living things are being genetically altered in more extensive ways than before, through techniques called synthetic biology. And some of the potential uses of genetically altered organisms, like releasing them to clean up pollution, would take place outside confined areas.

Biotechnology companies are trying to genetically engineer immune system cells so they attack tumors. But when these engineered cells are infused into patients, they sometimes cause a dangerous immune system overreaction. So some companies are now incorporating a suicide gene into the engineered cells, so they can be killed by administration of a drug if things go wrong.

A company called Oxitec has developed genetically engineered mosquitoes to fight mosquito-borne dengue fever. The altered mosquitoes have a gene that kills them unless they get the antibiotic tetracycline. They can be bred in the laboratory, where the antibiotic is available. But once they are released in the wild to mate with wild mosquitoes, they soon die, as do their offspring, which inherit the lethal gene, reducing the population of the insect.

The approaches in the two Nature papers were somewhat different, but both built on work done when Dr. Isaacs was a postdoctoral researcher in Dr. Church’s lab to create an organism that uses a genetic code slightly different from other life-forms.

All life on earth uses the same four chemical units of DNA, represented by the letters A, C, T and G, and generally the same 20 amino acids, the building blocks of proteins. A three-letter DNA sequence, or codon, tells the cell to incorporate a particular amino acid into a protein it is making. CTT, for instance, specifies leucine, and GTA valine. Three of the codons, however, tell the cell to stop construction of a protein.