Neuropilin-1 (NRP1) is a a 130-140 kDa transmembrane glycoprotein expressed by endothelial, dendritic, and regulatory T cells, as well as several other normal cell types and malignant tumor cells. NRP1 was first identified as a semaphorin (SEMA) receptor, involved in axonal guidance in embryonic development. NRP1 was also shown to act as a receptor for vascular endothelial growth factor (VEGF) and a promoter of angiogenesis through its interaction with VEGF-A165 (and other VEGFs) and the receptor tyrosine kinase (RTK) VEGF-R2. NRP1 plays versatile roles in angiogenesis, axon guidance, cell survival, migration, and invasion.