Fig. 1. Generation and phenotypic analysis of a hypomorphic Tbx1 mutant. (A) Gene targeting strategy used to generate the Tbx1^neo allele. (B) Generation of the loxP-flanked (flox) Tbx1^flox allele. (C) RT-PCR shows low amount of Tbx1 transcript expressed by the Tbx1^neo allele. The RNA was extracted from whole E10.5 embryos. (D-F) Alignment of the OFT in E18.5 embryos; in a Tbx1^+/+ embryo (D) the aorta (ao) communicates with the left ventricle (LV), in a Tbx1^neo/neo mutant (E) with correct alignment, the truncus (T) communicates with both left and right ventricles (RV). However, in the Tbx1^neo/neo embryo shown in F, the truncus communicates only with the RV (as in Tbx1^–/– embryos, not shown). (G-I) Intracardiac ink injection of E11.5 embryos shows normal OFT alignment in a Tbx1^+/+ embryo (G), lack of alignment in a Tbx1^–/– embryo (I), and an intermediate phenotype in a Tbx1^neo/neo embryo (H). av: atrioventricular valves; pt: pulmonary trunk; sv: semilunar valves; VSD: ventricular septum defect. Scale bars: 0.5 mm.