A wide spectrum of birth defects are caused by deletions of the DIGeorge syndrome critical region (DGCR) at human chromosome 22q11. Over one hundred such deletions have now been examined and a minimally deleted region of 300kb defined. Within these sequences we have Identified a gene expressed during human and murine embryogenesIs. The gene, named TUPLE1, and Its murine homologue, encodes a protein containing repeated motifs similar to the WD40 domains found In the beta-transducIn/enhancer of split (TLE) family. The TUPLE1 product has several features typical of transcriptIonal control proteins and In particular has homology with the yeast Tup1 transcrIptIonal regulator. We propose that haplolnsuffIcIency for TUPLE1 Is at least partly responsible for DIGeorge syndrome and related abnormalities.