Essential for nerve cord development. Functions downstream of fra to control axon guidance across the central nervous system (CNS) midline. In embryos expressed in the nerve cord, brain and overlying head tissue. Expression levels in the ventral nerve cord peak in stage 13 embryos. By stage 15 expression becomes restricted and appears to be expressed specifically in neurons which cross the midline. Expression levels decrease once most of the commissural axons have crossed the midline. Belongs to the commissureless family.