Ephrin-A4 is a member of the ephrin ligand family which binds members of the Eph receptor family. All ligands share a conserved extracellular sequence, which most likely corresponds to the receptor binding domain. Ephrin-A4 consists of approximately 125 amino acids and includes four invariant cysteines, It has been shown to bind EphA2, EphA3, EphA4, EphA5, EphA6, EphA7, and EphB1. Ephrin-A4 binds promiscuously Eph receptors residing on adjacent cells, leading to contact-dependent bidirectional signaling into neighboring cells. It may play a role in the interaction between activated B-lymphocytes and dendritic cells in tonsils.