MECP2 and Rare pervasive developmental disorder: Mutations in the gene encoding Mecp2 (methyl-CpG-binding protein 2) are known to cause the majority of cases (90–95%, Neul et al., 2008) of typical RTT and are also found in neuropsychiatric syndromes like autism, bipolar disorder with cognitive deficits and childhood-onset schizophrenia with intellectual disabilities.