Neuropilin-1 (Npn-1, previously neuropilin; also named CD304) is a 130 - 140 kDa type I transmembrane (TM) glycoprotein that regulates axon guidance and angiogenesis (1 - 4). The full-length 922 amino acid (aa) rat Npn-1 contains a 623 aa extracellular domain (ECD) that shares 98% aa identity with mouse and 93% aa identity with human, equine, bovine and canine Npn-1 (3, 4). The ECD contains two N-terminal CUB domains (termed a1a2), two domains with homology to coagulation factors V and VIII (b1b2) and a MAM (meprin) domain (c). In mouse and human, splice variants that lack the TM domain have been described and are either proven or presumed to be soluble antagonists (1, 5 - 7). The sema domains of Class III secreted semaphorins such as Sema3A bind Npn-1 a1a2 (8). Heparin, the heparin-binding forms of VEGF (VEGF₁₆₅, VEGF-B and VEGF-E), PlGF (PlGF2), and the C-terminus of Sema3 bind the b1b2 region (8, 9). Npn-1 and Npn-2 share 48% aa identity within the ECD and can form homo- and hetero-oligomers via interaction of their MAM domains (1). Neuropilins show partially overlapping expression in neuronal and endothelial cells during development (1, 2). Both neuropilins act asco-receptors with plexins, mainly plexin A3 and A4, to bind class III semaphorins that mediate axon repulsion (10). However, only Npn-1 binds Sema3A, and only Npn-2 binds Sema3F (1). Both areco-receptors with VEGF R2 (also called KDR or Flk-1) for VEGF₁₆₅ binding (1). Sema3A signaling can be blocked by VEGF₁₆₅, which has higher affinity for Npn-1 (11). Npn-1 is preferentially expressed in developing or remodeling arteries (1, 2). Npn-1 is also expressed on dendritic cells and mediates DC-induced T cell proliferation (12).