Eph receptor tyrosine kinases mould the behaviour of many cell types by binding membrane-
anchored ligands, ephrins, at sites of cell–cell contact. Eph signals affect both of the
contacting cells and can produce diverse biological responses. New models explain how
quantitative variations in the densities and signalling abilities of Eph receptors and ephrins
could account for the different effects that are elicited on axon guidance, cell adhesion and
cell migration during development, homeostasis and disease.