TBX1 and 22q11.2 deletion syndrome: However, the frequency of TBX1 mutations remains rare in deletion-negative patients: Gong et al. identified only a few probable TBX1 mutations after studying 40 patients with DGS/VCFS phenotypes [4], and Zweier et al. found a single TBX1 mutation after examining 10 patients with 22q11.2DS phenotype [8].