Williams-Beuren syndrome (WBS) is a developmental disorder with multi-system manifestations caused by haploinsufficiency for contiguous genes deleted in chromosome region 7q11.23. The size of the deletion is similar in most patients due to a genomic duplication that predisposes to unequal meiotic crossover events. While hemizygosity at the elastin locus is responsible for the cardiovascular features, the contribution of other genes to the WBS phenotype remains to be demonstrated. We have identified a novel gene, TBL2, in the common WBS deletion. TBL2 is expressed as a 2.4-kb transcript predominantly in testis, skeletal muscle, heart and some endocrine tissues, with a larger ∼5-kb transcript detected ubiquitously at lower levels. TBL2 encodes a protein with four putative WD40-repeats. An alternatively spliced transcript in TBL2 introduces a novel second exon with an in frame stop codon. This mRNA encodes a 75 amino acid protein with 43 amino acids identical to TBL2 at the N-terminus and no known functional domain. The mouse homolog, <i>Tbl2</i>, shows 84% sequence identity at the nucleotide level and 92% similarity at the amino acid level. Comparison of the mouse and human sequences identifies a conserved region that extends upstream of the previously published sequence with an initiation codon common to both species that adds 21 amino acids at the N-terminus. The <i>Tbl2 </i>gene has been mapped to mouse chromosome 5 in a region of conserved synteny with human 7q11.23. Since haploinsufficiency has been shown for other WD-repeat containing proteins, hemizygosity of TBL2 may contribute to some of the aspects of the complex WBS phenotype.