RB1 and Williams syndrome: Significantly, near the balanced translocation t(5;6), we characterized a deletion that inactivated a gene whose human homolog, GTF2IRD1, encodes a widely expressed, multifunctional helix loop helix transcription factor that binds to pRb and is one of the 16 genes present in approximately 1.6 Mb interval commonly deleted in Williams-Beuren syndrome (WBS) (139).