Although the majority of patients with Beckwith-Wiedemann syndrome (BWS) have a normal karyotype, the study of those rare patients with a cytogenetic abnormality has given considerable insight into the genetics of this condition. The karyotypic abnormalities found include partial chromosome duplications of paternal origin and maternally derived translocations which usually involve the 11p15 region and provide one of the lines of evidence for the location of the BWS gene(s). Because the extent of the duplicated region in these patients is variable, the phenotypic expression of BWS is presumably due to the presence of a common duplicated region. Two unrelated patients with BWS were both noted to have a similar unbalanced t(5;11)(p15;p14) translocation. The parents in both families were unaffected but both fathers carried a balanced translocation involving the same chromosomes. Since the extent and nature of the duplication apparently determines the complex phenotypes seen in these patients, we undertook a detailed analysis of the extent of the triplicated region using fluorescent in situ hybrisation (FISH). Despite having markedly different phenotypes and presenting in disimilar ways the two patients had apparently identical duplication breakpoints.
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