Fig. 1. (A) Normal anatomy with the aorta (Ao) arising to the left and dorsal to the pulmonary artery (Pa). (B) Fgf8 mutants had frequent transposition with the Ao arising to the right and adjacent to the Pa (black arrowhead), and also an abnormal subclavian artery pattern (white arrowhead). (C) Less frequently there was persistent truncus arteriosus (TA). Note left branch pulmonary artery (LPa) arising from Ao to supply lung (Lu) (white arrowhead in C). In wild type (D), the Ao arises from the left ventricle (LV), while in Fgf8 mutant (E) the Ao arises from the right ventricle (RV). In another mutant (F), a single outflow tract arising from both the RV and LV over-rides a ventricular septal defect (VSD). Fgf8 mutants had atrial septal defects (white arrowhead H) or common atrium (CA) (I). In addition, although normally the right and left ventricles are separated from the right atrium (RA) and left atrium (LA) by the tricuspid (TV) and mitral valves (MV), respectively (G,H), several mutants had a single atrioventricular valve (I, black arrowheads). In addition to these abnormalities, ink injections/inspection of near-term embryos demonstrated a normal sized aortic arch in wild type (white arrowhead in J) although Fgf8 mutants often had hypoplastic (white arrowhead in K) or completely absent (black arrow in L) aortic arch, which is derived from the fourth arch artery. BC, braciocephalic; LCC, left common carotid; LS, left subclavian.