ThebiologicalactivitiesoftheneurotrophinsaremediatedbybindingtothedifferentmembersoftheTrkfamilytyrosinekinasereceptors.ThreeTrkfamilyproteins,TrkA,TrkBandTrkC,exhibitingdifferentligandspecificities,havebeenidentified.Attheproteinsequencelevel,Trksarehighlyconservedbetweenspeciesandtheproteinsexhibitcross-speciesactivity.