To cross or not to cross: alternatively spliced forms of the Robo3 receptor regulate discrete steps in axonal midline crossing.

How growth cones grow toward and then away from intermediate targets is a key issue in axon guidance. In this issue of Neuron, Tessier-Lavigne and colleagues demonstrate that two different spliced isoforms of the Robo3 receptor act sequentially in commissural neuron growth cones to mediate this process at the ventral midline in the vertebrate spinal cord.