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Medical students learning the anatomy of the human cardiovascular system recognize that the blood vessels are named mainly on the basis of luminal diameter, branching, position, and organ supplied. Students and physicians rely upon the general constancy of vascular determinants from one individual to another and take for granted that anatomy books will not go out of date. It is only when they learn that these vessels with their proper diameters and branches are formed in the embryo, mostly before the heart starts beating, that students begin to appreciate the true complexity of the genetic program that governs the development of the vascular system. This appreciation deepens when errors of the basic developmental plan are revealed as ‘vascular malformations.’

The genetic and molecular mechanisms that control the development of the vascular system have remained a mystery, until recently. Driven in part by …