The January 2010 issue of the New England Journal of Medicine has an excellent summary article on Williams Syndrome. A free 21 day trial membership can be started to download the entire article in pdf format.
"Williams–Beuren syndrome (also known as Williams' syndrome; Online Mendelian Inheritance in Man [OMIM] number, 194050), a multisystem disorder, is caused by deletion of the Williams–Beuren syndrome chromosome region, spanning 1.5 million to 1.8 million base pairs and containing 26 to 28 genes. Exactly how gene loss leads to the characteristic phenotype of Williams–Beuren syndrome is unknown, but hypoexpression of gene products is likely to be involved. Estimated to occur in approximately 1 in 10,000 persons,1 Williams–Beuren syndrome is a microdeletion disorder, or contiguous-gene-deletion disorder, that can serve as a model for the study of genotype–phenotype correlations and potentially reveal
. . ."