We are interested in understanding the molecular mechanisms of neural circuit assembly at the level of synapse formation. The general wiring of the nervous system is achieved through a sequence of developmental events that include neuronal migration, axon guidance, axonal layer specificity, synaptic specificity and activity-dependent modification of nascent synaptic circuits. Both anatomical and electrophysiological data suggest that synaptic connections form between specific synaptic partners at particular subcellular locations (Benson et al., 2001). We are exploring the molecular mechanisms that specify synaptic connectivity in C. elegans. In the last four years, we have labeled the synapses of 6 classes of C. elegans neurons with single-cell resolution in vivo. Our genetic analysis of these synapses has led to the identification of two Wnt family proteins that act as negative regulators of synapse formation1; a E3 ubiquitin ligase complex that is required for selective synapse elimination2; and a Netrin/UNC-6-DCC/UNC-40 pathway that coordinates axon guidance and synapse formation in two synaptic partners, thus resulting in the proper formation of a neural circuit3. In addition, we have six other ongoing projects that study various aspects of synaptic assembly and specificity in C. elegans. These data suggest that diverse mechanisms can modulate connectivity and generate specificity at the level of synapse formation. Synaptic connections can be specified by both positive and negative regulators of synaptogenesis, which can act either via contact-dependent mechanisms or as diffusible molecules. Furthermore, non-neuronal cells such as glial cells may play important roles in the assembly of neural circuits.