A rare X-linked genomic disorder associated with interstitial chromosomal duplications at Xq28 encompassing the MECP2 gene. In males the disease has characteristics of infantile onset hypotonia, severe global developmental delay, intellectual disability, progressive spasticity, seizures, gastrointestinal symptoms and recurrent respiratory infections. In females, the phenotype is more variable. The syndrome is due to Xq28 duplications (< 4 Mb) involving the dosage-sensitive gene MECP2. The pattern of inheritance is X-linked. The recurrence risk is significant if the duplication encompassing the MECP2 gene is inherited from the mother, but very low if the duplication is de novo. There is full disease penetrance in males and variable penetrance in females due to the level and type of X-inactivation.