We have cloned and characterized Gtf2i, the mouse homolog of human GTF2I (general transcription factor II-I), which encodes BAP-135, a target for Bruton's tyrosine kinase. GTF2I represents the telomeric and functional copy of a duplicated gene flanking the 2-Mb Williams-Beuren syndrome (WBS) common deletion at 7q11.23. GTF2I is deleted in WBS, while a truncated centromeric pseudogene (GTF2IP1) is not deleted. In mouse, there appears to be only a single locus, Gtf2i, which we mapped to mouse chromosome 5 in a region of conserved mouse-human synteny. Gtf2i is 87.7% identical to GTF2I at the nucleotide and 97% at the amino acid level and generates several alternatively spliced transcripts. The gene is widely expressed in adult tissues and equally in all areas of the brain. Gtf2i transcript is detectable in ES cells by RT-PCR and on Northern blots of tissues from 7-dpc embryos. A ubiquitous expression pattern is seen by Northern and tissue in situ hybridization studies of 14-dpc embryos.