After neurons become polarized, both axons and dendrites continue to develop and the connections (synapses) between neurons will form. The nervous system progresses from a large number of disconnected neurons to a network of neuronal circuitries capable of generating functional outputs. To form the functional circuits, not only is it necessary for the axons of presynaptic neurons to grow and navigate through often long distances to the correct region to meet their targets, but also the dendrites of postsynaptic neurons need to grow and elaborate into the right shape to receive and process synaptic inputs. While extensive studies over the past decade have identified many molecules underlying axonal outgrowth and navigation, molecular mechanisms that control dendrite development are less well understood. Given that dendrites differ from axons in many important aspects morphologically as well as functionally, it seems likely that specific mechanisms are employed for dendrite development. Recently, we identified a novel and evolutionarily conserved member of immunoglobulin (Ig) superfamily, Dendrite arborization and synapse maturation 1 (Dasm1), which specifically controls mammalian neuron dendrite, but not axon, development (Figure 2). The domain structure and the initial functional studies suggest that Dasm1 is a membrane receptor. Currently we are investigating the extracellular and the intracellular signaling pathways of Dasm1.