Rett syndrome is one of the leading causes of mental retardation and developmental regression in girls. It is characterized by a period of normal psychomotor development followed by the loss of acquired motor and communication skills, autistic features and stereotypic hand movements. Rett syndrome is the first pervasive developmental disorder with a known genetic cause. The majority of cases are caused by de novo mutations in an X-linked MECP2 gene. Its product, methyl-CpG-binding protein 2, plays an important role in the regulation of gene expression and chromatin structure. Because the neuropathology of Rett syndrome shares certain features with other neurodevelopmental disorders, a common pathogenic process may underlie these disorders. This makes Rett syndrome a prototype for the genetic, molecular, and neurobiological analyses of neurodevelopmental disorders.