It took Dr. Gantz about two months to load all the necessary genetic elements onto a plasmid, a viruslike circle of DNA that genetic engineers use to insert genes into chromosomes. Some 680 wild mosquito larvae were injected with the cargo-carrying plasmid. About half grew to adults, which were mated with wild mosquitoes, and the progeny were then screened to see if the injected plasmids had successfully found their way to the eggs or sperm of the original larvae.

To help track the progress of the experiment, Dr. Gantz engineered a color marker into the plasmid’s cargo, a gene that turned the mosquitoes’ eyes red. Thus the appearance of red-eyed mosquitoes would mean the drive and its cargo had successfully been inherited.

Olga Tatarenkova, a member of Dr. James’s team, picked up the task of screening 25,000 mosquito larvae. One Saturday morning in July, she found two larvae with red eyes. After checking with other members to verify her finding, she emailed Dr. James to tell him the good news.

Dr. James, Dr. Gantz and Dr. Bier plan to refine the mosquitoes’ genetics and conduct trials in cages. He hopes that eventually scientists in some country where malaria is endemic will invite them to conduct a field trial and will monitor it carefully to make sure there are no adverse affects, even though it’s hard to see any ecological downside to protecting the mosquitoes against the malarial parasite.

Dr. Dimopoulos, who has engineered mosquitoes to rev up their immune systems and reject the malaria parasite, plans to add a gene drive as Dr. James has done, and to seek the Zambian government’s approval for a trial in a large, greenhouselike enclosure he operates in southern Zambia.

Rendering the wild mosquito population immune to the malaria parasite may seem a fairly minimal and benign intervention. But no gene drives have yet been released into the wild, and biologists are keen to avoid surprises that might arouse public hostility to the novel technology.

A more likely mishap is that the gene drive and its cargo genes will start to develop mutations that impair their inheritance or that natural selection will favor other genes that overwhelm them. The biologists could respond by developing new gene drives and cargoes, but the process could become uneconomical, in which case the new approach would fail or offer just partial advantages.

Dr. James plans to proceed in careful stages with the knowledge and approval of local authorities. “This is the kind of technology where the first trial has to be a success,” he said.