ably, these patients harbor mutations in a specific gene or genes that result in the disease phenotype. Recently, specific mutations in TBX1 were identified in 3 nondeleted DiGeorge families, supporting the conclusion that DiGeorge syndrome can be caused by loss of Tbx1 function. 18 However, several other genes in the commonly deleted region have also been implicated as potential causes or modifiers of DiGeorge syndrome, and many nondeleted DiGeorge patients do not have mutations in the coding region of TBX1. 18–22 Perhaps mutations in noncoding regulatory regions affect TBX1 expression in these patients, or perhaps mutations in other genes can cause similar phenotypes.