MECP2 (methyl-CpG binding protein 2), an X-linked gene encoding the methyl-cytosine binding protein MeCP2, is associated with two severe neurological disorders, Rett syndrome (RTT) and MECP2 duplication syndrome (MDS), which result from loss and gain of function of MECP2, respectively. This evidence concerns the gene MECP2 and Rett syndrome.