For Maribel Rivera, the sound of an ice cream truck in her Los Angeles neighborhood is like torture. Just the sound of the music sends the 24-year-old woman into an uncontrollable raging tantrum.
"It's like she can't get this and she needs it. It's like the end of the world for her," said her sister, Mercedes Rivera.
The cause of Maribel's rage isn't just a bad temper. It's a rare genetic disorder called Prader-Willi Syndrome. The syndrome stems from a flaw on Maribel's chromosome 15. It causes intellectual impairments, short stature and occasionally violent behavior. It also profoundly affects hunger, creating a sense of never being satisfied. Maribel's hunger is unimaginable.
"She does not have what we have. We know when we're full. For her, it's not like that. She's always hungry. And you know we always have to watch it. Because wherever it may be, even if it's in the trash, she'll get it," Mercedes said.
Mercedes made a documentary -- simply called "Maribel" -- in which she describes her sister's unceasing hunger and its effects on the family. In one scene, Maribel wanders off to a hot dog stand and, when she thinks no one is looking, begins to beg strangers to buy her a hot dog.
Families of children with Prader-Willi must take drastic measures to keep their kids from literally eating themselves to death. Maribel is always under supervision.
Mercedes said the main concern at home is ensuring that all of the food in the house is locked up. "We have to lock the refrigerator. Any other food in the pantry, it's all locked," she said.
Stealing Cupcakes as a Toddler
Jim and Kit Kane's 24-year-old daughter, Kate, is also afflicted with the syndrome. And they take similar measures to control Kate's eating.
"We have our cabinets locked, we have our refrigerator locked, and I slept on the couch in the family room, because I knew she would come down in the middle of the night and try to get in the refrigerator," Jim Kane said.
When Kate was born, the Kanes said something seemed wrong right away. Her muscle tone was weak and, strangely, she had no interest in food.
"They wanted her to nurse, and she wouldn't. She didn't have the sucking mechanism or strength or whatever, and they labeled her a failure-to-thrive baby," said Kit.
That -- ironically -- is an early symptom of Prader-Willi. Those with the disorder have practically no appetite. The Kanes needed Kate to eat, and were elated when she finally found her appetite.
"She looked like a little Buddha baby, and we were so happy," her mom recalled.
But the eating didn't stop and began to get out of control. When she was 2 years old, Kate started stealing cupcakes at birthday parties. By age 3, Kate weighed 45 pounds -- 50 percent above the average. When she was 5, someone finally put a name to what was happening.
"I felt convinced that Kate had the Prader-Willi syndrome, even though her genetic test was normal," said Dr. Dan Driscoll, one of the nation's leading specialists on Prader-Willi and the one who gave the Kanes the bad news.
"I became very attached to the family," Driscoll said, "because you're delivering, well ... a life sentence."
The Prader-Willi Syndrome Association estimates there are 30,000 people in the United States with Kate and Maribel's condition. It strikes people regardless of culture or diet or background.
Thanks to her parents' hard work, Kate qualified for residency in a home for adults with Prader-Willi in Wisconsin. There she is monitored 24 hours a day and has lost 100 pounds over the past year.
Residents are trained in healthier living and can earn income doing manual labor.
ABC News' John Donvan met up with the Kanes recently when Kate was home for the holidays. The setting was a restaurant, where she demonstrated newfound self-control and ordered just salad.
But there was something odd about Kate's behavior. Her eyes wandered off mid-conversation. It became apparent that she was following the food going to other tables. Kate explained the hunger is always there.
Like Mirabel, Kate would lie to get food. She shoplifted. She sneaked into a neighbor's home for it. She hid food in her closet.
Today, kids born with Prader-Willi can get off to a better start. They can be diagnosed virtually at birth and start hormone therapy to assist in their development.
Driscoll is studying the brains of kids with Prader-Willi. He thinks research into how Prader-Willi Syndrome triggers this condition, what body chemicals are involved and how they interact could yield benefits for everyone who struggles with their weight.
"We think we're going to get major clues by understanding Prader-Willi Syndrome, which is one reason for obesity, that gives us a window of opportunity to go in and help to understand other causes of obesity and, eventually, to figure out what's broken and then how to fix it," Driscoll said.
But Kate and Maribel's ceaseless hunger is a terrible price to pay, even if they can help make a lasting contribution to researchers' understanding of the condition.
Maribel's mom -- who shares her daughter's name -- points out there are also blessings, however. "The blessing is we've learned so much from Maribel," she says.
"A lot of special persons [are] out there," she said. "We should all look and see what they have inside."