Fig. 2. Time-course deletion of Tbx1 dissects the mutant phenotype. (A) Phenotypes scored in E18.5 TgCAGG-CreER^TM;Tbx1^flox/- or TgCAGG-CreER^TM;Tbx1^flox/+ (indicated by an asterisk) embryos after in utero exposure to TM at the stages indicated. The penetrance of defects was complete, unless indicated otherwise. Aortic arch defects included interrupted aortic arch type B and aberrant origin of the right subclavian artery. OFT, cardiac outflow tract; PTA, persistent truncus arteriosus. (B) Scheme of crucial time intervals of Tbx1 requirement for the structures or organs indicated, estimated from the results shown in A. The left extremity of each bar is set at the latest TM injection time that causes abnormalities, and the right end of the bar is set shortly after the next injection time. The gray areas in each bar indicate time required for full excision of the floxed allele (left) and possible extension of the time span (right). The arrow indicates that we do not know the end-point of the crucial time for the development of the secondary palate.