In contrast to contiguous gene syndromes such as Williams syndrome, several other microdeletion syndromes have been shown recently to be caused mainly by haploinsufficiency of a single responsible gene such as MEF2C in 5q14.3 microdeletion syndrome [24] or SHANK3 in Phelan-McDermid syndrome [25]. This evidence concerns the gene SHANK3 and Phelan-McDermid syndrome.