Investigation of a cryptic interstitial duplication involving the Prader-Willi/Angelman syndrome critical region

A 3-year-old female referred with developmental delay, hypotonia and seizur es was found to have a cryptic interstitial duplication of the Prader-Willi /Angelman critical region (PWACR). Her clinical features form part of a com mon phenotype characteristic of PWACR duplications including developmental delay, behavioural problems and speech difficulties. Microsatellite analysi s showed that the duplication had arisen de novo,was maternal in origin and involved the entire 4-Mb PWACR between the common deletion breakpoints. Th e existence of cryptic rearrangements emphasises the need for molecular tes ts alongside conventional cytogenetics when investigating abnormalities inv olving this imprinted region.