The first step in the initiation of EPH-mediated signaling is the high affinity interaction between EPH receptors and ephrin (EFN) ligands of the same subclass located on closely opposed cell surfaces. Membrane bound EPHs bind to EFNs with high affinity and this results in the development of close contacts between the cells. This association between EPHs and EFNs on opposing cells is called trans-interaction and this close contact is required for signaling. This high affinity interaction usually result in contact-mediated repulsion. With some exceptions, B-type EPH receptors (EPHB1-B6) interact with the B-subclass ephrins (EFNB1-B3). EPH and EFN initially form a 1:1 high affinity heterodimer, where EFN inserts its extended loop into a channel at the surface of the receptor (Himanen et al. 2007, Himanen & Nikolov 2003, Himanen et al. 2001).