Fig. 1. Co-expression of Tbx1 and Pitx2 in the SHF and asymmetric Tbx1 expression in wild-type embryos. In situ hybridization in transverse and sagittal sections through the heart shows overlapping expression of Tbx1 (A,E,I) with Pitx2 (B,F,J) within the Nkx2.5 (C,G,K) and Isl1 expression domains (D,H,L). Co-expression of Tbx1, Pitx2, Nkx2.5 and Isl1 is shown in the left pharyngeal mesoderm (pm), the left horn of the sinus venosus (lsv) and the outflow tract (oft) up to the inner curvature of the common ventricle (cv) at E8.5 (arrowheads in A-H) and E9.5 (arrowheads in I-L), respectively. (M-O) Transverse serial sections (cranial to caudal) of E9 embryos, where the asymmetric Tbx1 expression on the left side of the pharyngeal mesoderm (arrowhead in N,O) is visible. The asymmetry is limited to the most caudal part of Tbx1 expression domain. This asymmetry is retained in the Pitx2^-/- mice (arrowhead in P). da, dorsal aorta; fg, foregut; lcv, left cardinal vein. Scale bars: 100 µm in A-H; 200 µm in I-L; 200 µm in M-P.