Abstract
Two newly discovered receptors, Slit and Roundabout (Robo), help determine how far axons in developing fruit flies travel after they cross the line between the fly's left and right sides. In a pair of papers in the December issue of Neuron, two independent teams report that flies have two proteins very similar to Robo, called Robo2 and Robo3. In two more papers in this week's issue of Cell, both teams describe how the Robo proteins on an axon's cell membrane not only keep the axon from recrossing the midline but also help determine the particular path an axon takes.
