Ephrins are cell surface molecules with a wide range of biological functions that are involved in both embryogenesis and adult life. This includes important roles in boundary formation, cell migration, repulsive axon guidance, and the regulation of neuronal growth cone development. The ephrin family consists of eight members. Ephrin-A1 through A5 are anchored to the cell membrane via GPI linkage, while ephrin-B1 through B3 are transmembrane proteins. The interaction between ephrin ligands and its cognate Eph receptors involve both forward and reverse signaling.Ephrin-A1 was originally isolated in conditioned media from HUVEC cells treated with TNF-α, and the gene was found to be induced by TNF-α in these cells. Ephrin-A1 is broadly expressed on endothelial cells and epithelial cells, but it can also be released as a soluble molecule. Ephrin-A1 and its primary receptor EphA2 play a vital role in normal angiogenesis and tumor neovascularization. Ephrin-A1 expressed on tumor cells inhibits tumor cell proliferation and migration, but also supports tumor growth by promoting angiogenesis. Ephrin-A1 is also expressed on resting CD4+ T cells and its expression is down regulated in asthma patients.