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One in 12,000 children is born with Rett syndrome, yet few people have heard of it.

The genetic condition almost exclusively affects women, causing them to deteriorate neurologically and physically.

The progression of the disease can be roughly divided into four phases.

During the first stage, from about six to eighteen months of age, a baby's development slows, he loses interest in play, stops making eye contact, begins to walk clumsily and makes repetitive hand movements.

The second stage, known as 'rapid destruction', begins between the ages of one and four years.

The child finds it increasingly difficult to communicate and learn and there is often a deterioration of other brain functions.

Symptoms include inability to control hands, sudden anxiety accompanied by screaming, unsteadiness, breathing problems, sleep problems, slow head growth and digestive problems.

The third stage, the 'plateau', begins between the ages of three and ten.

Limbs become weak, epilepsy may develop, and there may be weight loss and teeth grinding.

However, many parents say that children are less upset and show more interest in their environment.

The final phase can last decades. This usually results in severe bending of the spine – scoliosis – and loss of the ability to walk.

Almost all cases are caused by a mutation in the MECP2 gene, which prevents the nerve cells in the brain from working properly.

Currently there is no cure and only the symptoms are treated.

It is estimated that approximately three-quarters of patients will live to be fifty years old Rett UK. The National Institute of Health says it is not possible to make reliable estimates of life expectancy after age 40.

According to Rettsyndrome.orgData from the Natural History Study found that a girl with Rett has a 100 percent chance of living to age 10, a greater than 75 percent chance of living to age 30, and a chance of more than 50 percent to reach the age of 50.

But because of Rett's rarity, very little has been published about its life expectancy.