A summary of callosal projection phenotypes after expression of Kir2.1, TeNT-LC, and both of them in layer II/II pyramidal neurons of the somatosensory cortex. Reducing neuronal excitability and blocking synaptic transmission leads to a great reduction in callosal projections to the S1/S2 border regions, and simultaneous manipulation results in a more pronounced reduction in the projections to the border region. Whereas reducing excitability in callosal neurons causes an overshooting phenotype in callosal layer targeting, blocking synaptic transmission impairs normal callosal axon arborization in both S1 and the S1/S2 border regions. It should be noted that callosal axons with reduced excitability and impaired transmitter release exhibit the overshooting phenotype but not the poor arborization phenotype in the somatosensory cortex. Arrowheads indicate the S1/S2 border. WM, White matter.