Abstract-TBX1, encoding a T-box containing transcription factor, is the major candidate gene for del22q11.2 or DiGeorge syndrome, characterized by craniofacial and cardiovascular defects including tetralogy of Fallot and common arterial trunk. Mice lacking Tbx1 have severe defects in the development of pharyngeal derivatives including cardiac progenitor cells of the second heart field that contribute to the arterial pole of the heart. The outflow tract of Tbx1 mutant embryos is short and narrow resulting in common arterial trunk. Here we show by a series of genetic crosses using transgene markers of second heart field derived myocardium and coronary endothelial cells that a subdomain of myocardium normally observed at the base of the pulmonary trunk is reduced and malpositioned in Tbx1 mutant hearts. This defect is associated with anomalous coronary artery patterning. Both right and left coronary ostia form predominantly at the right/ventral sinus in mutant hearts, proximal coronary arteries coursing across the normally coronary free ventral region of the heart. We have identified Semaphorin3c as a Tbx1-dependent gene expressed in subpulmonary myocardium. Our results implicate second heart field development in coronary artery patterning and provide new insights into the association between conotruncal defects and coronary artery anomalies. (Circ Res. 2008;103:142-148.)Key Words: Tbx1 Ⅲ outflow tract Ⅲ coronary artery patterning Ⅲ heart development T he significant fraction of congenital heart defects affecting the arterial pole of the heart reflects the complex events underlying formation and septation of this region of the heart. 1 During normal development the myocardial outflow tract (OFT) forms from progenitor cells of the second heart field (SHF) situated in adjacent pharyngeal mesoderm. 2 Addition of these cells to the heart is coordinated with that of cardiac neural crest cells and OFT endothelial cells. 3 The cylindrical OFT is subsequently divided to generate the ascending aorta and pulmonary trunk concomitant with ventricular septation. 4 Cardiac neural crest cells are essential for OFT septation and SHF deployment: neural crest ablation in the chick results in OFT and right ventricular hypoplasia. 3,5 Direct ablation of the SHF leads to defects in heart tube extension resulting in pulmonary atresia and a failure of ventriculoarterial alignment. 6 Coronary arteries arise from a plexus of epicardially derived vessels that selectively invade the base of the aorta. 7,8 Proximal right and left coronary arteries connect to ostia positioned at the right and left aortic sinuses facing the pulmonary trunk by a process of coalescence of endothelial strands. 9 -11 Anomalies in coronary artery patterning are an important component of congenital heart defects and occur in isolation and in conjunction with defects in outflow tract development. Coronary artery defects include single ostium or abnormal ostium positioning, either in the aorta or pulmonary trunk, and are a significant cause of sudden cardiac death. 12 ...