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A striking characteristic of vertebrate embryos is their bilaterallysymmetric body plan, which is particularly obvious at the levelof the somites and their derivatives such as the vertebral column.Segmentation of the presomitic mesoderm must therefore be tightlycoordinated along the left and right embryonic sides. We showthat mutant mice defective for retinoic acid synthesis exhibitdelayed somite formation on the right side. Asymmetric somiteformation correlates with a left-right desynchronization ofthe segmentation clock oscillations. These data implicate retinoicacid as an endogenous signal that maintains the bilateral synchronyof mesoderm segmentation, and therefore controls bilateral symmetry,in vertebrate embryos.
1 Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP/Collège de France, BP 10142, 67404 Illkirch Cedex, Strasbourg, France. 2 Departments of Medicine and Molecular Biology, Center for Cardiovascular Development, Baylor College of Medicine, Houston, TX 77030, USA.
* These authors contributed equally to this work.
Present address: Beckman Institute, California Institute ofTechnology, Pasadena, CA 91125, USA.
To whom correspondence should be addressed. E-mail: dolle{at}igbmc.u-strasbg.fr (P.D.); julien{at}igbmc.u-strasbg.fr (J.V.)
Present address: Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA. 
To whom correspondence should be addressed. E-mail: 
