Williams-Beuren syndrome in monozygotic twins with variable expression.

Five sets of monozygotic (MZ) twins with Williams-Beuren syndrome (WBS) have been reported so far. We report on an additional pair of mz twins concordant for WBS but variable expression for the syndrome. Although both faces look different monozygosity of the twins was proven by DNA fingerprint analysis, HLA, and blood group pattern. Both girls had the typical facial appearance with strabismus. Both had developmental delay, mild supravalvular aortic stenosis (SVAS), hypoplasia of both pulmonary arteries and multiple peripheral pulmonary stenoses, and inguinal hernia. Unilateral renal agenesis was seen in one of the twins. In addition the pedigree pointed to a second disorder with probably autosomal dominant inheritance. Both twins had a cleft palate, but their father had cleft lip and the grandfather as well as the greatgrandfather had cleft lip/palate. Findings of linkage analysis in pedigrees with nonsyndromic oral facial cleft were taken to suggest that a major locus for nonsyndromal oral facial cleft is located on the distal portion of chromosome 6. Linkage studies could serve as a starting point to examine a locus associated with WBS. Our observation and reports on the literature support the hypothesis that WBS is a genetic disorder.