Rett syndrome is a rare neurodevelopmental disorder affecting 1:1,000 female children. Revision of the diagnostic criteria in 2010 reflected a growing understanding of the heterogeneity of the disorder.1 Slowed head growth in infancy, for example, is no longer a criterion for diagnosis. Classic Rett syndrome (RTT) is determined by regression of language and fine motor skills and all 4 main criteria: 1) partial or complete loss of purposeful hand skills, 2) partial or complete loss of acquired speech, 3) gait abnormalities (e.g., dyspraxic gait) or inability to walk, and 4) characteristic midline hand movements. Exclusionary criteria include brain injury such as trauma, infection, or neurometabolic disease and markedly abnormal development in the first 6 months. Atypical Rett syndrome is diagnosed when there are at least 2 of the 4 main criteria and 5 of the 11 supporting criteria categorizing the behavioral features, neuromotor findings, and physical symptoms. Abnormalities of the MECP2 gene are found in 95% of affected girls2 with other genes contributing to infrequent subtypes. Mutations of MECP2 may be found in children with other neurodevelopmental disorders such as autism. It is likely that …