Neurexins represent one of the most diverse protein families found in the central nervous system. The three mammalian Neurexin genes generate six primary transcripts, which are subjected to extensive alternative splicing. As a result, thousands of isoforms are generated. We are investigating the repertoire of Neurexin variants expressed by individual neurons and have developed methods to selectively ablate isoforms from identifiable subsets of cells in the developing mouse brain. The goal of these studies is to test whether Neurexin isoforms provide a code that determines specific connectivity, function, or morphology of cells and thereby contribute to the specification of cell identity in the CNS.