The Eph family of receptors comprises the largest known family of receptor tyrosine kinases. Ligands of Eph family receptors are structurally related membrane bound proteins that can be subdivided into two major subclasses, ephrin A and ephrin B. Expression of Eph receptors is tissue specific and appears to be tied to developmental events. Ligands in the ephrin A subclass, including the prototype family member ephrin A1 (B61), are membrane associated through glycosylphosphatidyl inositol linkages, whereas ephrin B subclass consists of ligands with transmembrane domains. The general role of the Eph family is in mediating repulsive cell-cell interaction.