Rett syndrome is a leading cause of postnatal neurodevelopmental regression. Rett syndrome is caused by mutations in MECP2, the gene encoding methyl-CpG binding protein 2. In up to 96% of all classic cases, Rett syndrome cases are caused by mutations or deletions in MECP2. The phenotypic spectrum of MECP2 mutations is broad and includes mental retardation with or without seizures, Angelman syndrome-like phenotype, and autism. Mecp308/Y mice carry a truncating mutation and display many of the features seen in Rett syndrome. Social behavior abnormalities and impaired social interactions in Mecp308/Y mice suggest that MeCP2 plays a role in modulating the activity of genes and neurons important for social interactions. Mice that overexpress MeCP2 at twice the endogenous levels develop a progressive neurologic disorder, demonstrating that MeCP2 levels are tightly regulated and raising the possibility that duplications or gain-of-function mutations of MECP2 might underlie some cases of neurodevelopmental X-linked disorders.