Receptor for netrin unc-6 required for asymmetric axon formation, axon guidance and cell migrations (PubMed:16520734, PubMed:8861903, PubMed:11454756, PubMed:22426253, PubMed:9473333). Required during axon formation, in response to unc-6, to initiate, maintain and orient asymmetric neuronal growth, and may signal via phosphoinositide 3-kinase (PI3K) (PubMed:16520734). Mediates axon attraction of neuronal growth cones in the developing nervous system upon ligand binding (PubMed:8861903, PubMed:11454756, PubMed:22426253, PubMed:9473333). Axon migration is mediated by the secreted unc-6, which promotes attraction of neurons and axons through binding to the unc-40 receptor, while repulsion requires both unc-5 and unc-40 receptors (PubMed:8861903, PubMed:11454756, PubMed:22426253, PubMed:9473333). Involved in dendritic morphogenesis; may act by localizing unc-6 at the tips of growing dendrites for interaction with unc-5 on the apposing branch to induce mutual repulsion (PubMed:22426253). Involved in the ventral-dorsal and anterior-posterior migration of gonadal distal tip cells (DTCs) along the body (PubMed:11454756). Involved in the positioning of ray 1, the most anterior ray sensilium, in the male tail (PubMed:24004945). Positively modulates a TGF-beta-like signaling pathway, independent of its role in unc-6 mediated axon guidance, in association with RGM drag-1 (PubMed:24004951). Mediates attraction of muscle plasma membrane extensions, known as muscle arms, towards the secreted madd-4 guidance cue, enhanced by interaction with the coreceptor eva-1 (PubMed:25122090). Interacts with netrin unc-6 (PubMed:22426253, PubMed:8861903). Interacts with eva-1 (PubMed:25122090). Interacts (via fibronectin type-III domain) with drag-1 (PubMed:24004951). Expressed on the surface of motile cells and pioneering axons (PubMed:8861903). Expressed at the onset of gastrulation (at about 100 minutes (min) after first cell cleavage) on the surface of all cells, and then gradually diminishes (PubMed:8861903). Highly expressed in the neurula (at about 400 min), on ventral cord motorneurons, including cell bodies and axons, undergoing axonogenesis (PubMed:8861903). Expressed in stage L1 larvae in ventral epidermoblasts as they undergo planar movements within the epithelium and in neuroblasts QL and QR and their descendants as they migrate longitudinally along the epidermis (PubMed:8861903). Expressed in L2 and later stage larvae in distal tip cells (DTCs) of hermaphrodites as they migrate along the body wall (PubMed:8861903). Expressed in the DTCs beginning at the time of the ventral-to-dorsal second migration (PubMed:11454756). Expression on the immature hermaphrodite specific neuron (HSN) is localized to the ventral surface beginning at the early L2 stage (PubMed:16520734). Fibronectin type-III 6 motif is essential for the role of unc-40 in a TGF-beta-like signaling pathway. Belongs to the immunoglobulin superfamily. DCC family.