Pervasive developmental disorders (PDDs) largely overlap with the autism spectrum disorders (ASDs). PDDs manifest in childhood and mainly affect social interaction, including communication and behavior. PDDs can be caused by mutations in genes involved in brain development and function, environmental insults, or the combination of environmental factors and genetic susceptibility. The pervasive developmental disorder pathway that Reactome has annotated is the "Loss of function of MECP2 in Rett syndrome". For review of this topic, please refer to Picket and London 2005, Currenti 2010, Elsabbagh et al. 2012, Ferreri 2014.