The gene GTF2IRD1 is localized within the critical region on chromosome 7 that is deleted in Williams syndrome patients. Genotype-phenotype comparisons of patients carrying variable deletions within this region have implicated GTF2IRD1 and a closely related homolog, GTF2I, as prime candidates for the causation of the principal symptoms of Williams syndrome. We have generated mice with an nls-LacZ knockin mutation of the Gtf2ird1 allele to study its functional role and examine its expression profile. In adults, expression is most prominent in neurons of the central and peripheral nervous system, the retina of the eye, the olfactory epithelium, the spiral ganglion of the cochlea, brown fat adipocytes and to a lesser degree myocytes of the heart and smooth muscle. During development, a dynamic pattern of expression is found predominantly in musculoskeletal tissues, the pituitary, craniofacial tissues, the eyes and tooth buds. Expression of Gtf2ird1 in these tissues correlates with the manifestation of some of the clinical features of Williams syndrome.