less ▁( comm ), ▁a ▁protein ▁involved ▁in ▁Rob o ▁re ceptor ▁tra ff ick ing . ▁Comm ▁prevents ▁Rob o ▁from ▁reaching ▁the ▁cell ▁memb rane ▁by ▁target ing ▁the ▁re ceptor ▁for ▁the ▁end os om al ▁path way ; ▁this ▁allows ▁the ▁growth ▁cone ▁of ▁the ▁comm iss ural ▁ax on ▁to ▁cross ▁the ▁mid line ▁by ▁prevent ing ▁Rob o / Sl it ▁rep uls ive ▁interactions . ▁Comm ▁expression ▁turns ▁off ▁after ▁the ▁growth ▁cone ▁has ▁crossed ▁the ▁mid line ; ▁this ▁perm its ▁Rob o / Sl it ▁rep ulsion ▁and ▁prevents ▁the ▁growth ▁cone ▁from ▁crossing ▁the ▁mid line ▁again . ▁ ▁Ver te br ates , ▁on ▁the ▁other ▁hand , ▁do ▁not ▁possess ▁a ▁comm ▁hom olog ; ▁instead ▁they ▁facil itate ▁mid line ▁crossing ▁through ▁alternative ▁sp lic ing ▁of ▁Rob o 3 ▁( aka . R ig - 1 ). ▁Rob o 3 ▁has ▁two ▁is o forms , ▁ 3 . 1 ▁and ▁ 3 . 2 , ▁and ▁these ▁is o forms ▁interact ▁with ▁Rob o 1 ▁and ▁Rob o 2 ▁( Rob o 1 / 2 ) ▁through ▁c is ▁interactions ▁at ▁the ▁leading ▁edge ▁of ▁the ▁growth ▁cone . ▁Before ▁crossing ▁the ▁mid line ▁Rob o 3 . 1 ▁in hib its ▁Sl it / Rob o ▁rep uls ive ▁signal ing , ▁allowing ▁the ▁comm iss ural ▁ax on ▁to ▁cross ; ▁after ▁crossing ▁the ▁mid line ▁Rob o ▁ 3 . 1 ▁is ▁replaced ▁by ▁Rob