Ephrin-B2, also known as Htk-L, ELF-2, LERK-5, and NLERK-1, is a 40 kDa member of the Ephrin-B family of transmembrane ligands that bind and induce the tyrosine autophosphorylation of Eph receptors. The extracellular domains of Ephrin-B ligands are structurally related to GPI-anchored Ephrin-A ligands. Eph-Ephrin interactions are widely involved in the regulation of cell migration, tissue morphogenesis, and cancer progression. Ephrin-B2 preferentially interacts with receptors in the EphB family (1, 2). Mature zebrafish Ephrin-B2 consists of a 201 amino acid (aa) extracelluar domain (ECD), a 21 aa transmembrane segment, and an 87 aa cytoplasmic domain (3). Within the ECD, zebrafish Ephrin-B2 shares 56% aa sequence identity with human, mouse, and rat Ephrin-B2. Ephrin-B2 is expressed presynaptically on neurons (4, 5). It promotes presynaptic development, EphB2 shedding, axonal growth cone collapse, and neurite repulsion, and also regulates inflammatory and neuropathic pain (4 - 6). Ephrin-B2 is expressed by vascular mural cells and arterial vascular and lymphatic endothelium (7, 8). It exerts proliferative and migratory effects on these cells during angiogenesis and lymphangiogenesis in part by regulating the signaling activity of VEGF R2 and VEGF R3 (7 - 9). Ephrin-B2 plays a role in the immune response by mediating monocyte extravasation and T cell costimulation (10, 11). It is upregulated in invasive cancers and promotes tumor cell migration, invasion, and tumor angiogenesis (12 - 14). It functions as a cellular entry receptor for Hendra and Nipah viruses (15). Ephrin-B2 is also important for the separation of the urinary and intestinal tracts during development (16).