SUPERNUMERARY RING CHROMOSOME-20 CHARACTERIZED BY FLUORESCENCE IN-SITU HYBRIDIZATION

We report on a boy with mild dysmorphic features and developmental del ay, in whom karyotyping showed an additional minute ring chromosome in 60% of metaphases. Fluorescence in situ hybridization (FISH) with a c entromere specific probe demonstrated that the ring chromosome contain ed the centromeric region of chromosome 20. The ring was highlighted c ompletely using a chromosome 20 painting probe. A cosmid probe for 20p 12-13 gave a positive signal and hybridization with an all-telomere pr obe showed one signal, suggesting a breakpoint in the 20p telomere. Th e results suggested that only a small part of 20q was involved in this ring. The ring was also detected in 18% of nuclei of a buccal smear. The phenotypic similarities of symptoms in the proband to patients wit h a (partial) trisomy 20p and the dissimilarities to symptoms in patie nts with (partial) trisomy 20q were in agreement with the FISH results .