+ C ▁content ▁and ▁a ▁sp lice ▁accept or ▁site ▁that ▁is ▁the ▁signal ▁for ▁trans - sp lic ing . ▁Such ▁a ▁trans - sp lice ▁site ▁is ▁essentially ▁defined ▁as ▁an ▁accept or ▁( 3 ') ▁sp lice ▁site ▁without ▁an ▁up stream ▁don or ▁( 5 ') ▁sp lice ▁site . ▁▁ ▁In ▁e uk ary otes ▁such ▁as ▁e ug len oz o ans , ▁din of lag ell ates , ▁sp ong es , ▁n emat odes , ▁c n id ari ans , ▁c ten op hor es , ▁flat w orm s , ▁cr ust ace ans , ▁cha et ogn ath s , ▁rot if ers , ▁and ▁t unic ates , ▁the ▁length ▁of ▁sp lic ed ▁leader ▁( SL ) ▁out rons ▁range ▁from ▁ 3 0 ▁to ▁ 1 0 2 ▁nucle ot ides ▁( nt ), ▁with ▁the ▁SL ▁ex on ▁length ▁r anging ▁from ▁ 1 6 ▁to ▁ 5 1 ▁n t , ▁and ▁the ▁full ▁SL ▁R NA ▁length ▁r anging ▁from ▁ 4 6 ▁to ▁ 1 4 1 ▁n t . ▁ ▁Process ing ▁ ▁In ▁standard ▁c is - sp lic ing , ▁the ▁don or ▁sp lice ▁site ▁in ▁up stream ▁position ▁is ▁required ▁together ▁with ▁an ▁accept or ▁site ▁located ▁on ▁down stream ▁position ▁on ▁the ▁same ▁pre - R NA ▁mole c ule . ▁By ▁contrast , ▁the ▁SL ▁trans - sp lic ing ▁re lies ▁on ▁a ▁ 3 ' ▁accept