
What's the cause?
Virtually all cases of Rett Syndrome (RTT) are caused by a mutation (alteration) in the methyl CpG binding protein 2 gene (MECP2). The MECP2 gene is on the X chromosome and has the information for the synthesis of a protein, the methyl-CpG-binding protein 2, MeCP2.

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MeCP2 is a multifunctional protein. One of the functions of the protein is act as one of the “switches” that can increase gene expression or tell others genes when to turn off. Loss of function of this protein, because of the mutation in the MECP2 gene, may lead to inappropriate overexpression of some other genes.
The MeCP2 protein is essential for brain development and its expression rises after birth, which could explain the delayed onset of RTT in early childhood.

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If you want to know more about MECP2 gene and MeCP2 protein, please check the following link: