DiGeorge syndrome is mainly caused by a multigene, heterozygous, interstitial chromosomal deletion. Of the approximately 30 deleted genes, Tbx1 is the only gene that, after an extensive functional analysis in the mouse, has been found to be haploinsufficient. The mutant phenotype is convincingly similar to the human syndrome, and its human homolog, TBX1, is the only gene for which mutations have been found in some patients without the chromosomal deletion. The research interest in this syndrome is driven not only by the obvious clinical significance of the disease but also by a broader biological importance. In particular, this syndrome is the most typical developmental defect of the embryonic pharyngeal system: a transient, vertebrate-specific structure that contributes to diverse tissues of the head, neck and thorax. Many birth defects, including a large fraction of congenital heart disease cases, derive from developmental problems of the pharyngeal system. Tbx1 is an excellent tool to probe the genetic network governing embryonic pharyngeal development.