During development in animals with binocular vision, retinal ganglion cell axons from each eye navigate through the optic nerve to the optic chiasm. Here, some axons from the retinal area near the temple project to ipsilateral targets; those from other retinal areas project to contralateral targets. Herrera and colleagues now report that the transcription factor Foxd1 plays a dual role in the formation of the mouse optic chiasm (see p. 5727). Their examination of eye development in Foxd1-null embryos, and of co-cultures of Foxd1-deficient retinas and chiasms, reveals that Foxd1 loss disrupts the expression of Zic2 and EphB1, which direct ipsilateral projection. Surprisingly, many fibres from all over the mutant retinas project ipsilaterally or stall at the optic chiasm. Thus,Foxd1, which is expressed in the retina and the chiasm, helps to specify the outer, ventrotemporal area of the mouse retina and to pattern the brain region where the optic chiasm forms.