Rett syndrome (RTT) is a neurological disorder that causes severe cognitive and physical impairments. In its classic form, it appears to affect only females, with an incidence of up to one in every 10,000 live female births. Its cause is most often a mutation in the methyl-CpG binding protein-2 (MeCP2) gene, located on the X chromosome at Xq28. However, although a MeCP2 mutation is found in most cases of the classic form, RTT remains a clinical rather than a molecular diagnosis. MeCP2 mutations have not been found in all cases of RTT, and the mutation has been found in individuals who do not meet the clinical diagnostic criteria for classic or variant RTT. This entry describe the behavioral phenotype of RTT based on a systematic ...