They say their findings could lead to new treatments for obesity.
The team studied an Israeli Arab family whose members were severely obese. They found that the family members had a mutation in a gene that produces a protein called CEP19.
When the researchers deleted this gene in mice, the rodents became obese and developed diabetes. They also had increased appetites and burned less energy.
Scientists note, however, that research done in animals often fails to produce similar results in humans.
The study was published online Nov. 21 in the American Journal of Human Genetics.
"Starting with gene discovery in a single family with morbid obesity, these studies led to the identification of a gene that seems to be fundamental to regulating nutritional status [body weight]," study co-senior author Dr. John Martignetti, of the Icahn School of Medicine at Mount Sinai, in New York City, said in a journal news release.
"This gene is shown to be present not only in humans and mice, but also in the simplest known single-cell animal," he said. "Nature considers this gene so important that it has preserved its structure for more than 700 million years."
The specific role of the CEP19 protein in maintaining a balance between leanness and obesity remains unknown. Further research is needed to determine how the protein affects appetite control, the amount of calories the body burns and insulin sensitivity, the study authors said.
Previous research has indicated that genes play a role in 40 percent to 90 percent of cases of obesity, according to the news release.
The U.S. National Heart, Lung, and Blood Institute has more about overweight and obesity.