"I've seen some children that can talk and communicate a little," Wyler said. "And I've seen others that are totally laid out. They have no movement. They can't do anything by themselves. Literally, if they're 8 years old, it's like taking care of a baby."

Tarby saw the first "Fumarase child" in the community 15 years ago. He said the oldest victim is now about 20 years old. In March 2000, Tarby co-authored an article in the medical journal "Annals of Neurology" describing eight new cases of Fumarase Deficiency in the Southwest. It has now grown to 20 known cases in the polygamist community on the Utah-Arizona border.

Tarby said children suffering from Fumarase Deficiency have unusual facial features and frequent "grand mal" epileptic seizures. The children require constant care from parents and close relatives. "In some ways, they are really kind of remarkable people," Tarby said. "They do treat these kids pretty well."

Wyler agreed that the parents and close relatives are loving caregivers. He said it's partly because they believe it's a calling from God. "They would just assume they've been given a test and they need to pass this test," Wyler said. "And it's their lot in life to take care of a child like this. And they'll give it everything they've got. And they'll do a good job. Very good job."

Tarby said the early founder who brought the recessive gene into the community had numerous children, so copies of the gene were passed on to children and grandchildren. When cousins or other close relatives marry, two copies of the gene can be passed on to a single child, triggering the disease.

In the FLDS community, marriages with cousins and even closer relatives are common, according to Bistline. "There are people that have married their nieces," Bistline said. "People who have married their aunts."

It's all part of the community's religious system, according to Wyler. "Well, around here, of course, when you get married, you're told who to marry and when to get married and things like that. So, that's really not going to change, I don't believe."

"As long as they've got the leadership they've got," Bistline said, "they'll never change."

It's believed that more than half the residents carry the recessive gene. That means the number of cases will likely grow. Tarby said there could be hundreds of victims in coming generations. "No, it wouldn't surprise me," Tarby said. "Wouldn't surprise me."

Wyler hopes FLDS leaders will change their marriage practices. "Now that they know there's a problem," Wyler said, "they need to quit sweeping it under the rug and pretend there's not a problem. And (they should) say, 'OK, now you know when you cross these certain lines together, then this happens.' And they need fresh blood."