Williams syndrome (WS) (Williams et al. 1961; Beurenet al. 1962) is a multisystem disorder characterized by mental retardation, a specific neurobehavioral profile,characteristic facies, infantile hypercalcemia, cardiovascular abnormalities, progressive joint limitation, hernias, and soft skin. Recent studies have shown that hemizygosity at the elastin (ELN) gene locus on chromosome7q is associated with WS (Ewart et al. 1993). Furthermore, two FISH studies using cosmid recombinants containing the 5' or the 3' end of the ELN gene revealed deletion of the entire ELN gene in 90%-96% of classical WS cases (Lowery et al. 1995; Nickerson et al. 1995). However, the size of the 7ql 1.23 deletions and the mechanism by which these deletions arise are not known.