▁target ; ▁round about ▁( Rob o ) ▁re cept ors ▁and ▁their ▁lig and , ▁Sl it , ▁are ▁a ▁well - stud ied ▁example ▁of ▁rep uls ive ▁guidance . ▁Rob o ▁re cept ors ▁were ▁initially ▁identified ▁in ▁D ros oph ila ▁mel an og aster ▁using ▁a ▁forward ▁gen etic ▁screen ▁to ▁search ▁for ▁mole cules ▁involved ▁in ▁mid line ▁crossing ▁at ▁the ▁floor ▁plate . ▁Rob o / Sl it ▁loss - of - function ▁mut ations ▁result ▁in ▁ax ons ▁crossing ▁the ▁mid line ▁multiple ▁times , ▁whereas ▁gain - of - function ▁results ▁in ▁little ▁to ▁no ▁mid line ▁crossing ; ▁consequ ently ▁this ▁interaction ▁was ▁determined ▁to ▁be ▁important ▁in ▁prevent ing ▁non - comm iss ural ▁ax ons ▁from ▁crossing ▁the ▁mid line ▁and ▁comm iss ural ▁ax ons ▁from ▁rec ross ing . ▁ ▁How ▁the ▁comm iss ural ▁ax on ▁regul ates ▁its ▁response ▁to ▁Sl it ▁rep ulsion ▁at ▁the ▁mid line ▁has ▁been ▁extens ively ▁studied ▁in ▁both ▁D ros oph ila ▁and ▁verte br ates ; ▁in ▁these ▁two ▁models ▁the ▁growth ▁cone ' s ▁response ▁to ▁Sl it ▁has ▁been ▁shown ▁to ▁be ▁reg ulated ▁through ▁re ceptor ▁tra ff ick ing ▁and ▁alternative ▁sp lic ing , ▁respectively ▁( Fig . 3 ). ▁ ▁Re ceptor ▁tra ff ick ing ▁is ▁used ▁extens ively ▁throughout ▁growth ▁cone ▁migration ; ▁in ▁D ros oph ila ▁prior ▁to ▁crossing ▁the ▁mid line ▁these ▁neur ons ▁express ▁comm iss ure