Ephrins and their receptors EphR play an important role in cell communication in normal physiology, as well as in disease pathogenesis. Binding of the ephrin (Eph) ligand to EphR requires cell-cell contact, since both molecules are anchored to the plasma membrane. The resulting downstream signals occur bidirectionally in both EphR-expressing cells (forward signaling, depending on Eph kinase activity) and ephrin-expressing cells (reverse signaling). Eph signaling controls cell morphology, adhesion, migration and invasion. Ephrins can be subdivided into 2 groups, A and B, depending on their respective receptors EphA or EphB. The nine human EphA receptors bind to five GPI-linked ephrin-A ligands. Interactions are promiscuous within each class, and some Eph receptors can also bind to ephrins of the other class. All ephrin As contain a highly conserved receptor binding ectodomain described by this model. Although ephrin As do not have a cytoplasmic tail (in contrast to ephrin Bs), they are still capable of downstream activation of Src family kinases and phosphoinositide-3-kinases, most likely involving coreceptors such as neurotrophin receptors.