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The development of axonal tracts requires interactions between growth cones and the environment. Major bundles, particularly in the internal capsule, are completely defective in mice with constitutive mutation of Celsr3. In order to understand better how Celsr3 controls axonal tract formation, we generated a conditional allele that allowed inactivation of Celsr3 in different sectors of the forebrain. Effects of Celsr3 inactivation specifically in the telencephalon, in the ventral forebrain, or in the cortex, demonstrate essential roles for the gene, both in the neurons that project their axons to subcerebral targets such as the spinal cord, as well as in cells that guide projecting axons through the ventral forebrain. These observations provide unequivocal in vivo evidence that heterotypic interactions between axons and guidepost cells govern axonal path formation in mammals, and that Celsr3 plays a key role in this process. In absence of cortico-subcortical connections, mice can survive up to P20, allowing studies of behavior and cortical maturation. Mutant mice with defective corticospinal tracts survive normally and provide a model to evaluate in vivo the role of this tract in motor function in rodents.