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Axon guidance defects in Vax1-mutant mice. Shown here is an immunohistochemical analysis of wild-type and Vax1-mutant embryos at E18.5 labeled with antibodies against the cell adhesion protein L1 (brown). In wild-type mice (top), retinal ganglion cell (RGC) axons enter the brain at the base of the hypothalamus and cross to the contralateral side at the X-shaped optic chiasm. In Vax1-mutant mice (bottom), axons do not form an optic chiasm and do not penetrate the brain. They instead terminate in encapsulated whorls. This defect results from the loss of Vax1 in the midline guidance cells with which RGC axons must normally interact at the optic chiasm. (For details, see Bertuzzi et al., p. 3092; see also Hallonet et al., p. 3106).