The Prader-Willi syndrome (PWS) and the Angelman syndrome (AS) are human 
neurogenetic disorders involving the imprinting mechanism, at the 15q11-13 
chromosome region. The predominant genetic defects in PW are 15q11-13 deletions 
of paternal origin and maternal chromosome 15 uniparental disomy. In contrast, 
maternal deletions and paternal chromosome 15 uniparental disomy are associated 
with a different neurogenetic disorder, the AS. In both disorders, these 
mutations are associated with parent-of-origin specific methylation at several 
15q11-13 loci. We studied 5 patients suspect of PWS and 4 patients suspect of AS 
who were referred to the Medical Genetics Unit at the University Hospital of 
Medical School from Ribeirão Preto. Our objective was to establish the correct 
clinical and etiological diagnosis in these cases. We used conventional 
cytogenetics, methylation analysis with the probe KB17 (CpG island of the SNRPN 
gene) by Southern blotting after digestion with the Xba I and Not I restriction 
enzymes. We studied in patients and their parents the segregation of the (CA)*** 
repeats polymorphisms by PCR, using the primers 196 and IR4-3R. All the patients 
had normal conventional cytogenetical analysis. We confirmed 3 cases of PWS: one 
by de novo deletion, one by maternal chromosome 15 uniparental disomy and one 
case with no defined cause determined by the used primers. We confirmed 2 cases 
of AS, caused by de novo deletion at the 15q11-13 region, and one case with 
normal molecular analysis but with strong clinical characteristics.