Neuropilin-1/CD304 is a type-I transmembrane protein. It forms a heterodimeric complex with vascular endothelial growth factor receptor 2, and this complex functions as a receptor for vascular endothelial growth factor. On the other hand, Neuropilin-1/CD304 forms a complex with plexin-1, and this complex acts as a semaphorin 3A receptor. Neuropilin-1/CD304 is involved in angiogenesis, axon guidance, cell survival, migration, and invasion. Recently, it has been reported that Neuropilin-1/CD304 is an essential component located on the immunological synapse and acts as a potent cell-surface marker for murine regulatory T cells (Treg). Neuropilin-1/CD304 expressed by Treg cells promotes long interactions between Treg cells and immature dendritic cells and restricts access of effector cells to the antigen-presenting cells.