To the naked eye, the egg of the Anopheles gambiae mosquito is just a dark speck, but under a 100-power microscope, it shows up as a fat, slightly curved cucumber, somewhat narrower at one end. In the wild, it is typically found in shallow, sunlit puddles in sub-Saharan Africa, but it can survive in any number of wet places at around 80 degrees Fahrenheit. In a laboratory in London, behind three sets of locked doors enclosing negative-pressure containment vestibules, Andrew Hammond, a doctoral student in molecular genetics, picks up a clump of Anopheles eggs on a small paintbrush and lines them up on a microscope slide. Hammond looks for the narrow end, where the germ line cells that will form the next generation are located. With delicate nudges of a joystick, he maneuvers a tiny needle through his field of vision until it just penetrates the egg membrane, and the click of a button releases a minute squirt of DNA. Whether the genetic material reaches and binds to its target region is then a matter of luck, and luck is, generally, with the mosquito. Hammond’s success rate, of which he is very proud, is around 20 percent.

…

Still, there are voices calling to proceed slowly. “If we were to intentionally set out to cause the extinction of a species, we should think about that,” says Henry Greely, a Stanford law professor and bioethicist. “I would want there to be some consideration and reflection, and a social consensus, before we take that step.” His argument is based partly on the slippery slope: If mosquitoes, then why not rats? “I’m not sure I care if mosquitoes suffer, if they can suffer. But mammals or birds, I do care.”