 The risk to siblings is <1% if the affected child has a gene deletion or uniparental disomy, up to 50% if the affected child has a mutation of the imprinting control region, and up to 25% if a parental chromosomal translocation is present.Prenatal testing is possible for any of the known genetic mechanisms.A microdeletion in one family of the snoRNA HBII-52 has excluded it from playing a major role in the disease.