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		INTRODUCTION The development of the face differs from the establishment of all other structures in that the facial muscle and bone are of neural crest rather than mesodermal origin. Cephalic neural crest cells arising in the region between the mid-diencephalon and the eighth rhombomere migrate into specific branchial arches and give rise to mesenchymal cells which eventually establish bone, cartilage, connective tissue, and adipose tissue of the facial region.1The first branchial arch forms the frontonasal process and mandible, while the second branchial arch gives rise to the hyoid cartilage of the neck.2,3 Much of the interest in craniofacial development has centered around the potential role of the neural crest in patterning. However,Sonic hedgehog(shh), a signaling molecule involved in patterning a number of structures, most notably the limb4,5and tooth6, has recently been shown to be expressed in the craniofacial region. Initially, it is expressed in the axial mesendoderm, and later in the ectoderm of the branchial arches and of the frontonasal and maxillary processes.7,8If SHH signaling is decreased or eliminated by gene mutation or treatment with anti-SHH antibody or the plant alkaloid cyclopamine, defects are formed in the craniofacial region, particularly loss of midline structures. 9-11 Short range SHH signaling in vertebrate limb development is regulated by a feedback loop. SHH induces the expression of fgf4in the apical ectodermal ridge. In return, FGF4 sustains shhexpression.5,12,13A modified feedback loop is also seen in the developing lung where FGF10 is thought to induce expression of shh.14We were interested in whether such a feedback loop existed in the craniofacial region, where another member of the FGF family, fgf8, is expressed in the ectoderm of the frontonasal process and the branchial arches.15,16