How do wiring molecules specify cortical connections?

The laminar and columnar organization of the cortex is reflected in the projections to and from the cortex and in the intracortical connections. During development of the cerebral cortex, growing axons are able to distinguish between the different cortical layers, and cortical axons originating from different laminae respond to different layer-specific signals. Here we consider some experimental systems for identifying mechanisms that contribute to the elaboration of layer-specific cortical connections.