er ic ▁copy ▁SM N 2 . ▁In ▁health y ▁individuals , ▁the ▁SM N 1 ▁gene ▁codes ▁the ▁surv ival ▁of ▁motor ▁neur on ▁protein ▁( SM N ) ▁which , ▁as ▁its ▁name ▁says , ▁plays ▁a ▁cru cial ▁role ▁in ▁surv ival ▁of ▁motor ▁neur ons . ▁The ▁SM N 2 ▁gene , ▁on ▁the ▁other ▁hand ▁– ▁due ▁to ▁a ▁variation ▁in ▁a ▁single ▁nucle ot ide ▁( 8 4 0 . C → T ) ▁– ▁under go es ▁alternative ▁sp lic ing ▁at ▁the ▁j unction ▁of ▁intr on ▁ 6 ▁to ▁ex on ▁ 8 , ▁with ▁only ▁ 1 0 – 2 0 % ▁of ▁SM N 2 ▁trans cript s ▁coding ▁a ▁fully ▁functional ▁surv ival ▁of ▁motor ▁neur on ▁protein ▁( SM N - fl ) ▁and ▁ 8 0 – 9 0 % ▁of ▁trans cript s ▁resulting ▁in ▁a ▁trunc ated ▁protein ▁comp ound ▁( SM N Δ 7 ) ▁which ▁is ▁rapidly ▁de grad ed ▁in ▁the ▁cell . ▁ ▁In ▁individuals ▁affected ▁by ▁S MA , ▁the ▁SM N 1 ▁gene ▁is ▁mut ated ▁in ▁such ▁a ▁way ▁that ▁it ▁is ▁unable ▁to ▁correctly ▁code ▁the ▁SM N ▁protein ▁– ▁due ▁to ▁either ▁a ▁delet ion ▁occurr ing ▁at ▁ex on ▁ 7 ▁or ▁to ▁other ▁point ▁mut ations ▁( fre qu ently ▁resulting ▁in ▁the ▁functional ▁conversion ▁of ▁the ▁SM N 1 ▁sequence ▁into ▁SM N 2 ). ▁Al most ▁all ▁people , ▁however , ▁have ▁at ▁least