TBX1 and Dravet syndrome: To test the hypothesis that this variation might be controlled by an environmental factor such as ID, we crossed male C57BL/6 J background mice heterozygous for either a Tbx1 null allele80 (a model of 22q11.2 deletion syndrome), or the Dp1Tyb duplication81 (a model of DS), with ID females and analysed embryonic phenotypes at E15.5.