Syndrome characterized by neurobehavioral disorders, hypotonia, cognitive deficit, language delay and seizures. Prevalence is unknown. The clinical picture is highly variable even within the same family. Paternal duplications are rarely symptomatic (developmental delay/ behavioral disorders). The syndrome is due to interstitial duplications that encompass the imprinted Prader-Willi/Angelman critical region, of which deletions lead to Prader-Willi and Angelman syndromes. The causative genes are imprinted and expressed from the maternal allele.