tbx-2(bx59) mutates a conserved histidine residue within the dimerization domain of the T-box to a tyrosine (H145Y; accession CCD69847), and mutations affecting this domain in human TBX1 result in gain-of-function associated with some cases of DiGeorge and velocardiofacial syndromes [49]. This evidence concerns the gene TBX2 and 22q11.2 deletion syndrome.