Out of the millions of species of insects, only about a hundred suck human blood. Now, scientists say they’ve figured out how one mosquito became a vampire: a gene that makes it particularly sensitive to human odor.

This is one of the few situations in which researchers have pinned down a gene underlying a complex behavior, and their results may point the way to thwarting this potentially deadly insect. “The more we can understand about how mosquitoes sense human odors, the better we will be at designing repellents and baits,” says Carolyn McBride, the evolutionary neurobiologist at Princeton University who led the work.

Curious about how insects developed a taste for humans, McBride focused her efforts on the Aedes aegypti mosquito, which has spread worldwide. The insect, which passes on the dengue, yellow fever, and chikungunya viruses, lives side by side with a very close relative along the coast of Kenya. The relative—known as A. aegypti formosus, or “forest form” of A. aegypti—buzzes through forests, doesn’t attack humans, and lays its eggs in water in tree holes and rock pools. The domestic form, A. aegypti aegypti, meanwhile, thrives in homes, laying eggs in water containers. The domestic and forest forms can interbreed, but for the most part they avoid each other.

McBride confirmed another intriguing difference between the two insects. When she and colleagues brought them back to the lab, they found that the forest mosquito preferred to dine on guinea pigs, whereas its domestic cousin liked humans.

The team compared the activity of all the genes in the antennae of the two forms. There were many differences and one gene that was much more active in the domestic mosquito: Or4, which codes for an odor receptor. The researchers plugged Or4 into fruit fly nerve cells that lack the Or4 receptor and tested which odors the nerve cell could pick up on that it couldn’t before. The strongest response was to an odor molecule called sulcatone, McBride and her colleagues report online today in Nature. Many organisms release sulcatone, but humans reek of it, producing four times the amount the researchers found in live chickens or in horse, cow, and sheep hair. In guinea pigs, this odor was undetectable.

When the scientists took at closer look at the Or4 gene, they found that its sequence varied in the two types of Kenyan mosquitoes. The gene comes in seven different versions, three of which predominate in the human-biters. Thus, human-biting mosquitoes have distinct versions of the gene and make a lot more of its odor-sensing protein than do the forest mosquitoes.

"The paper is well designed, and the experiments lead us nicely from demonstrable behavioral differences right the way through to the identification of a gene which may be associated with such differences,” says James Logan, an entomologist at the London School of Hygiene & Tropical Medicine, who was not involved with the work.

“To present such a convincing case for a single gene being the causative factor is rare,” adds Jeffrey Powell, an evolutionary geneticist at Yale University, who was not involved with the work. Although there are likely to be other factors involved in human preference, “it seems the one identified is a, if not the, major factor,” he says.

According to Logan, A. aegypti is not controlled by bed nets or insecticides, yet spreads serious diseases such as dengue fever. The work, he says, "could have important implications for mosquito control measures in the future," for example in helping develop new baits or repellents.