Two patients with classical features of Angel-man syndrome (AS) and one with 
Prader-Willi syndrome (PWS) had unbalanced reciprocal translocations involving 
the chromosome 15 proximal long arm and the telomeric region of chromosomes 7, 8 
and 10. Fluorescence in situ hybridization (FISH) was used for the detection of 
chromosome 15(q11-13) deletions (with probes from the PWS/AS region) and to 
define the involvement of the telomere in the derivative chromosomes (with 
library probes and telomere-specific probes). The 15(q11-13) region was not 
deleted in one patient but was deleted in the other two. The telomere on the 
derivative chromosomes 7, 8 and 10 was deleted in all three cases. Thus, these 
are true reciprocal translocations in which there has been loss of the small 
satellited reciprocal chromosome (15) fragment.