Genetic imprinting has been implicated in the etiology of two clinically 
distinct but cytogenetically indistinguishable disorders--Angelman syndrome (AS) 
and Prader-Willi syndrome (PWS). This hypothesis is derived from two lines of 
evidence. First, while the molecular extents of de novo cytogenetic deletions of 
chromosome 15q11q13 in AS and PWS patients are the same, the deletions originate 
from different parental chromosomes. In AS, the deletion occurs in the 
maternally inherited chromosome 15, while in PWS the deletion is found in the 
paternally inherited chromosome 15. The second line of evidence comes from the 
deletion of an abnormal parental contribution of 15q11q13 in PWS patients 
without a cytogenetic and molecular deletion. These patients have two maternal 
copies and no paternal copy of 15q11q13 (maternal uniparental disomy) instead of 
one copy from each parent. By qualitative hybridization with chromosome 15q11q13 
specific DNA markers, we have now examined DNA samples from 10 AS patients (at 
least seven of which are familial cases) with no cytogenetic or molecular 
deletion of chromosome 15q11q13. Inheritance of one maternal copy and one 
paternal copy of 15q11q13 was observed in each family, suggesting that paternal 
uniparental disomy of 15q11q13 is not responsible for expression of the AS 
phenotype in these patients.