Test Details: Three distinct neurodevelopmental disorders arise primarily from deletions or duplications that occur at the 15q11-q13 locus: Prader-Willi syndrome (PWS), Angelman syndrome (AS) and 15q11-q13 duplication syndrome (Dup15q syndrome). FISH helps in detection of AS/PWS caused by lack of expression of maternal or paternal genes at 15q11-q13. 70% cases result from deletion of 15q11-q13 region on one homologue of chromosome 15.