▁at ▁ty ros ine ▁resid ues . ▁E ph rin - B ▁also ▁contains ▁a ▁P D Z ▁binding ▁mot if ▁important ▁in ▁ax on ▁guidance ▁reg ulation ▁via ▁G - prote in ▁signal ing . ▁Re verse ▁signal ing ▁can ▁also ▁occur ▁when ▁E ph rin - A ▁is ▁activ ated ▁by ▁E ph - A 3 ▁binding ; ▁this ▁is ▁reg ulated ▁by ▁metal lo prote ase - dependent ▁cle av age ▁of ▁E ph rin - A . ▁▁▁ ▁E ph / E ph rin ▁bid irection al ▁signal ing ▁is ▁important ▁for ▁ax on ▁guidance ▁and ▁target ▁selection ; ▁mapping ▁of ▁ret inal ▁ax ons ▁along ▁anterior - post er ior ▁axis ▁in ▁the ▁visual ▁system ▁is ▁reg ulated ▁by ▁E ph rin - A / E ph - A ▁medi ated ▁rep ulsion . ▁In ▁the ▁t ect um , ▁the ▁trans cription ▁factor ▁Eng ra iled ▁creates ▁an ▁E ph rin - A ▁concentration ▁gradient ▁along ▁the ▁anterior - post er ior ▁axis ; ▁this ▁results ▁in ▁different ▁signal ing ▁c ues ▁to ▁the ▁growth ▁con es ▁which ▁also ▁express ▁grad ed ▁levels ▁of ▁the ▁re ceptor . ▁The ▁top ographic ▁location ▁of ▁ret inal ▁ax ons ▁along ▁d ors al - vent ral ▁axis ▁requires ▁both ▁forward ▁and ▁reverse ▁signal ing ▁by ▁E ph rin - B / E ph - B ▁gradient ▁medi ated ▁att raction . ▁ ▁As ▁described ▁above ▁for ▁the ▁visual ▁system , ▁E ph / E ph rin ▁signal ing ▁plays