An acronym CATCH 22 (for cardiac defects, abnormal face, thymic hypoplasia, cleft palate and hypocalcemia) has been proposed to describe the commonly affected structures in DGS (Figure 2). Reviews of DGS diagnosed clinically have generally emphasized the variability of each of these features, such that the most immunodeficient patients may have perfectly normal hearts. Even patients diagnosed with chromosome 22qll-qter deletions are phenotypically variable. Concurrence of two or more of the following probably suffices to make a clinical diagnosis of DGS: conotruncal heart defect, hypocalcemia, hypoplastic/absent thymus and typical facial dysmorphism (including small palpebral fissures, a 'beaked' or 'bulbous' nose and small or posteriorly rotated ears).

Features of DGS overlap with other malformations affecting the migration of cephalic neural crest cells. These include the velocardiofacial (VCF; Shprintzen) syndrome, the CHARGE association and various combinations of midline structural defects found in association with hypertelorism (e.g. Optiz-Frias syndrome, Kallmann syndrome). Thymic hypoplasia has also been reported with abnormalities of chromosome 10 and observed in fetuses exposed in utero to 13-c/s-retinoic acid (Accutane®). In the mouse, a DGS phenocopy that includes thymic hypoplasia can be produced by recessive knockout of hox 1.5 in transgenic embryos. There are no hox gene homologs known on human chromosome 22. The DGS pheno-

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