Rett syndrome affects one in every 10,000 to 15,000 live female births. It occurs in all racial and ethnic groups worldwide. Prenatal testing is available for families with an affected daughter who has an identified MECP2 mutation. Since the disorder occurs spontaneously in most affected individuals, however, the risk of a family having a second child with the disorder is less than 1 percent. Genetic testing is also available for sisters of girls with Rett syndrome to determine if they are asymptomatic carriers of the disorder, which is a rare possibility.
Girls have two X chromosomes, but only one is active in any given cell.
Some of the child's brain cells use the healthy gene and express normal amounts of the proteins. The story is different for boys who have the MECP2 mutation. Because boys have only one X chromosome they lack a back-up copy that could compensate for the defective one, and they have no protection from the harmful effects of the disorder. Boys with the defect die shortly before or after birth. Doctors diagnose Rett syndrome by observing signs and symptoms during the child's early growth and development, and conducting ongoing evaluations of the child's physical and neurological status.