Axon guidance - A Roundabout way of avoiding the midline

Because we're bilaterally symmetrical, we need to be able to coordinate motor control on both sides of the body. To do this, many neurons in the central nervous system project their axons across the so-called midline to the opposite side of the body. But why do only some axons do this while others remain on the same side? This system of control relies on previously identified long-range attractive signals called netrins, and — it now turns out — also on short-range repulsive signals that belong to products of the roundabout family. Moreover, these repulsive signals are downregulated by members of another gene family called commissureless.