Williams-Beuren syndrome (WBS) is a rare genetic disorder (1/20000-50000) and is usually caused by a 1.5- to 1.8-Mb heterozygous deletion on chromosome 7q11.23. At least 25 genes have been identified in the deletion region in WBS patients, which is flanked by large low-copy-repeat sequences (>320 kb). By using FISH as well as microsatellite analysis, it is not possible to get a precise identification of the size of the deletion. For determining the deletion size, we developed a reliable quantitative PCR approach. Our assay screens 2.5 Mb of the WBS region in 100- to 300-kb intervals. This methodology has been tested in DNA samples of 65 patients with the clinical suspicion of WBS. In every case we were able to exclude or to identify the presence of a deletion and to determine its size. Deletion size varied from 0.2 Mb to 2.5 Mb. The 2.5-Mb rearrangement represents the largest deletion described at present and it was detected in a severely affected patient. We report the detection efficiency of this new system and the genotype/phenotype correlation.

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