Table 2.

Molecular defects and recurrence risks in AS

Genetic defect	Proportion of cases [13]	Recurrence risk
De novo deletion of 15q11q13 on the maternal chromosome	75%	<1%a
Paternal uniparental disomy (UPD) of chromosome 15	1–2%	<1%b
Imprinting defect (without an imprinting centre deletion)	3%	<1%
Imprinting centre deletion	≈10–15% of patients with an imprinting defect	50% (if present in a non-mosaic state in the mother)
UBE3A mutation	5–10%	50% (if present in a non-mosaic state in the mother)
No identifiable molecular abnormality	10–15%	Unknown (up to 50%)

^aIf maternal karyotype is normal

^bIf parental karyotypes are normal