Neuroscientists have long been fascinated by the mechanisms by which the nervous system has evolved to enable it to manage activities on each side of the body. During development, as axons reach the midline, specialised cells regulate whether they cross to the opposite side or migrate longitudinally along the same side.1 This is well-illustrated in animals with binocular vision, where the crossing of retinal ganglion cell (RGC) axons at the optic chiasm is regulated by a group of developmental axonal guidance molecules called the ephrins and the Eph receptors.2

Eph receptors comprise the largest known family of receptor tyrosine kinases, with at least 14 members in mammals. Based on the structure of their extracellular domain and ligand binding specificity, Ephs are divided …