Rett syndrome is caused by MECP2 gene mutation at the time of conception.

In October 1999, it was discovered by researchers at the Baylor College of Medicine in Houston that mutation of the  MECP2 gene is the main cause of Rett syndrome. To date, mutation of the MECP2 gene has been found in 95% of individuals who have been diagnosed with the disorder. It is not known why these mutations occur.

The MECP2 gene contains instructions which enable genes to function properly. If the MECP2 gene is not functioning properly, it allows genes to turn on or off at inappropriate times, thus disturbing the precisely regulated sequence necessary for proper development of a human’s central nervous system.

RETT Syndrome Association of Australia