The mechanisms involved in connecting axons to specific targets during the development of a precise neuronal network have been unclear. In September 26 Nature, Philippe Monnier and colleagues at Migragen AG, Tuebingen, Germany, show that a repulsive guidance molecule provides positional information for temporal axons invading the optic tectum (Nature, 419:392-395, September 26, 2002).

Monnier et al. cloned and characterized a chicken tectal membrane-associated glycoprotein — which they named repulsive guidance molecule (RGM). They observed that at low nanomolar concentration RGM induced collapse of temporal but not of nasal growth cones in the eye and was repulsive to axons from the temporal half of the retina.

"The present identification and characterization of RGM is important for our understanding of retinotectal map formation, and will help to elucidate the molecular mechanisms of axon guidance," conclude the authors.

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