The mechanisms that ensure pairing of axons with the appropriate target cells during CNS development are unknown. We are using the highly reproducible connectivity of the mouse cerebellum as a model system to understand the signaling events between afferents and cells in the target area that ensure the formation and/or stabilization of synapses with the correct postsynaptic partners. Using mixed culture assays of specific pre- and postsynaptic source we have identified several signaling systems that can regulate afferent growth and selective synapse formation. The function of these signaling systems in rodents in vivo is tested by conditional knock-out approaches and in utero electroporation which enable us to manipulate gene expression in a cell-type specific manner with high temporal control. The long-term goal of these studies is to understand the molecular mechanisms that control synaptic specificity in the CNS.