Williams-Beuren syndrome (WBS) is a microdeletion syndrome caused by haploinsufficiency of genes at 7q11.23. Here we describe the identification and characterization of a novel gene named GTF2IRD1, for GTF2I-repeat domain 1, within the WBS deletion region. Northern blot analysis revealed ubiquitous expression during development with two transcripts of 3.6 kb and 5.0 kb generated by alternative splicing. GTF2IRD1 encodes a protein of 944 amino acids that contains a region of high similarity to a unique motif with helix-loop-helix forming potential occurring within the transcription factor GTF2I. Analogous to TFII-I, the product of GTF2IRD1 may have the ability to interact with other HLH-proteins and function as a transcription factor or as a negative transcriptional regulator. A recent report of the identification of a muscle-specific transcription factor, MusTRD1, supports this hypothesis (O'Mahoney et al., 1998). The open reading frame described for MusTRD1 is identical to that of GTF2IRD1; however, the putative MusTRD1-protein is 486 amino acids shorter than the predicted protein encoded by GTF2IRD1. A heterozygous deletion of GTF2IRD1 may contribute to the complex WBS phenotype.