Summary of CST axon guidance in WT and knockout animals and a model for plexin signaling in CST axon turning. (a) Diagrams of cross sections of brainstem and spinal cord summarizing CST axon guidance defects observed in adult mutant mice. Ventrolateral CST axons were only observed in plexin and Sema6A mutants. The Sema6A-/- phenotype was relatively diverse between animals. Summarized in this diagram are the major defects. (b) Model for motor CST axon turning at the pyramidal decussation (refer to Discussion). C, caudal; D, dorsal; Pyr Dec, pyramidal decussation; R, rostral; SpC, spinal cord; V, ventral.