Narrator: This is Science Today. The main function of the region of the brain called the hypothalamus is homeostasis – the regulation of the body's status quo, including food intake and body weight. Christian Vaisse, an assistant professor of medicine at the University of California, San Francisco, is studying a brain protein expressed in the hypothalamus called the melanocortin-4 receptor, which is already known to play a role in the urge to eat.

Vaisse: In 1998, we actually described the first mutation in the melanocortin 4 receptor in humans and it turns out about 2.5 percent of all severely obese humans have mutations in the melanocortin-4 receptors.

Narrator: Recently, Vaisse discovered a new group of mutations in this receptor that affects the body's baseline level of activity.

Vaisse: This is a protein that is implicated in the long-term regulation of energy balance. So basically, what it helps do is adapt your energy intake, which is your food intake, to your energy expenditure.

Narrator: For Science Today, I'm Larissa Branin.