The tight structure of Rachelle’s days, combined with the lack of food anywhere, led to a complete absence of her tantrums. Once, after an argument over cards, she put her head on the table and refused to participate in therapy. Another time, she was clipping ads from the paper, a favorite activity, and said she didn’t want to go to class. Cherpes sat down across from her and watched as Constance Casey, a staff member, encouraged her to go earn her points.

“I’ll keep your stuff here for you,” she told her. “I’ll make sure nobody bothers it.” A moment later, Rachelle got up and walked toward her classroom.

“Good choices!” Cherpes said.

The Prader-­Willi program at the Children’s Institute of Pittsburgh began in 1982 with a single patient. Until 2012, when HealthBridge Children’s Hospital in Houston started a program, the institute was the only place in the country, and one of few in the world, to offer inpatient treatment for the syndrome. It now houses roughly 10 patients at a time — both adults and children — for stays of four to 10 weeks, depending on their medical needs and what an insurer will cover. The cost of care, per patient per day, averages about $1,500. Over the past few years, patients seem to be coming to the institute sicker than before, sometimes arriving in wheelchairs and on gurneys — a fact that the staff attributes to insurance companies’ increasing their requirements for inpatient rehabilitation while misunderstanding the disorder and what it takes to treat it. “As soon as they hear ‘obesity,' ” says Kwaj Overton, the nurse case manager, “they think, Well, they need to try Curves.”

Actually, much of what the institute does — restricting patients’ caloric intake and getting them to exercise — would help any obese person lose weight. For people with Prader-­Willi, though, it is impossible to make those changes without also managing grave health complications and out-of-control behavior, which is hard to untangle from exposure to food. People with the syndrome can resist raiding an unlocked cabinet, especially in the company of others. But this effort at control often erupts later in aggression. The institute’s philosophy is that eliminating any exposure to food — except at rigidly scheduled mealtimes for which menus are posted weeks in advance — is the only way patients can forget their hunger and anxiety so they can enjoy other food-free activities: crafts, gardening, dancing. There are no punishments for acting out, only incentives for participating. “If the words are ‘You have to do this,’ maybe I do or maybe I don’t, but I’m going to test you and see,” Cherpes said. “In a battle of wills, the person with Prader-­Willi is always going to win.”

A major part of the program is teaching parents how to create a similar environment at home. The staff members also talk to schools about making accommodations, including securing classmates’ lunchboxes, finding a place for a child to eat alone outside the cafeteria and providing a one-on-one aide for transitions between classrooms — measures that administrators, especially those in strapped districts, often resist. Over all, the results appear to be positive and lasting. Patients discharged in 2012 lost an average of 27.5 pounds during their stays, and 96 percent continued or sustained their weight loss after 18 months. Nine months out, a vast majority had not needed emergency medical or psychiatric treatment.

For obesity researchers, the uniqueness of Prader-­Willi presents a paradox: The causes of obesity in the general population are far more complex and varied than those of the syndrome, but Prader-­Willi’s specific genetic roots, while not expected to explain all forms of obesity, could offer universal truths about the biology of hunger and fullness. Robert Nicholls, a geneticist at the Children’s Hospital of Pittsburgh who studies Prader-­Willi, believes it is “very unlikely” that our species evolved multiple, separate systems to govern eating. Understanding why people with Prader-­Willi switch in early childhood from an extreme lack of interest in food to insatiability could offer clues about the nature of appetite — which might eventually help scientists minister to a person’s specific type of overeating or prevent it altogether. It’s possible that it could do the same for conditions of undereating and malnutrition, like anorexia. Whether damage to the hypothalamus can be undone at all, regardless of its cause, is still an open question, but one that a successful treatment for Prader-­Willi could answer. “Taking that genetic condition — if that is curable, there could be hope there,” says Joan Han, an endocrinologist at Le Bonheur Children’s Hospital in Memphis, Tenn., who conducts research at the National Institutes of Health. “It may not tell us everything about obesity in general, but it’s an essential part of the big picture.”

One promising avenue of exploration illuminated by Prader-­Willi involves a gene disrupted by the syndrome, Magel2. During the last 10 years, Rachel Wevrick, the University of Alberta geneticist, and colleagues discovered that mice lacking the gene develop many of the same symptoms that people with Prader-­Willi have, including behavioral abnormalities, low muscle tone and obesity. And though they are less active than normal mice, they do not eat less, so they gain excessive amounts of fat. By analyzing samples of their brain tissue, Wevrick found that the ability of their hypothalamus to sense the leptin produced by the body’s fat cells was impaired, disabling a neural pathway that tells the animals whether they need to conserve energy and consume more calories. Within the past year, researchers have also identified more than a dozen children in whom only the Magel2 gene on the chromosome 15 segment is mutated. At least several of the children have abnormally large appetites — all have low muscle tone — reinforcing the idea that the gene may play a central role in producing those same Prader-­Willi symptoms. The Rhythm drug, RM-493, uses a synthetic protein to bypass the neural pathway (in humans) that should be governed by leptin but does not work in the mice that lack the gene. When a similar compound is given to those mice, they eat less food.