Corticospinal tract (CST) defect at the pyramidal decussation of PlxnA4 mutants. (a-h) Post-mortem unilateral DiI placement into the motor cortex of P4 wild-type (a, c, e, g) and PlxnA4 mutant brains (b, d, f, h). Traced CST axons in wild-type project past the pons and decussate normally (a, c, e) and enter the dorsal funiculus (arrowhead in (g)). Tracings at this age in PlxnA4 mutants show no apparent misguided axons at the MHB (b, d), but the same defect at the level of the decussation as described for the L1-staining at P10, with many ventrolaterally projecting axons (arrows in f, h) and a few correctly decussating axons (arrowhead in h). Scale bar: 200 μm.