COLORADO CITY, Arizona - In a dusty neighborhood under sheer sandstone cliffs studded with juniper on the Arizona-Utah border, a rare genetic disorder is spreading through polygamous families on a wave of inbreeding.

A sign marks the town of Hildale, Utah, home to the nation's largest polygamist community - the Fundamentalist Church of Jesus Christ of Latter Day Saints (FLDS), a polygamist sect that broke from the mainstream Mormon church 72 years ago, in this photo taken May 31, 2007. [Reuters]

The twin border communities of Hildale, Utah, and Colorado City, Arizona, have the world's highest known prevalence of fumarase deficiency, an enzyme irregularity that causes severe mental retardation brought on by cousin marriage, doctors say.

"Arizona has about half the world's population of known fumarase deficiency patients," said Dr. Theodore Tarby, a pediatric neurologist who has treated many of the children at Arizona clinics under contracts with the state.

"It exists in a certain percentage of the broader population but once you get a tendency to inbreed you're inbreeding people who have the gene there, so you markedly increase the risk of developing the condition," he said.

The community of about 10,000 people, who shun outsiders and are taught to avoid newspapers, television and the Internet, is home to the Fundamentalist Church of Jesus Christ of Latter Day Saints (FLDS), a sect that broke from the mainstream Mormon church 72 years ago over polygamy.

The group, who wear conservative 19th-century clothing, is led by Warren Jeffs, who was arrested in August and charged as an accomplice to rape for using his authority to order a 14-year-old girl against her wishes to marry and have sex with her 19-year-old cousin.

Doctors in the area declined requests for interviews and families refuse to talk to reporters. But former FLDS members, independent doctors and authorities say the disorder appears to have struck at least 20 children in the past 15 years.

"The disease itself is very rare in the rest of the world," said Dr. Vinodh Narayanan of Arizona's St. Joseph's Hospital & Medical Center and Barrow Neurological Institute. Doctors worldwide had only studied about 10 cases just a decade ago.

"Once you get people within in the same community marrying, then the chances grow of having two people carrying the exact same mutation."

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