▁plays ▁an ▁important ▁role ▁in ▁the ▁reg ulation ▁of ▁cell ▁cycle . ▁ ▁It ▁has ▁been ▁demonstrated ▁that ▁two ▁is o forms ▁of ▁IL - 1 5 ▁m R NA ▁are ▁generated ▁by ▁altern atively ▁sp lic ing ▁in ▁m ice . ▁The ▁is o form ▁which ▁had ▁an ▁alternative ▁ex on ▁ 5 ▁containing ▁another ▁ 3 ’ ▁sp lic ing ▁site , ▁exhib ited ▁a ▁high ▁transl ational ▁efficiency , ▁and ▁the ▁product ▁lack ▁hydro ph ob ic ▁domains ▁in ▁the ▁signal ▁sequence ▁of ▁the ▁N - ter minus . ▁This ▁suggests ▁that ▁the ▁protein ▁derived ▁from ▁this ▁is o form ▁is ▁located ▁int rac ell ul ary . ▁The ▁other ▁is o form ▁with ▁normal ▁ex on ▁ 5 , ▁which ▁is ▁generated ▁by ▁integral ▁sp lic ing ▁of ▁the ▁alternative ▁ex on ▁ 5 , ▁may ▁be ▁released ▁ext rac ell ul ary . ▁ ▁Although ▁IL - 1 5 ▁m R NA ▁can ▁be ▁found ▁in ▁many ▁cells ▁and ▁t issues ▁including ▁m ast ▁cells , ▁cancer ▁cells ▁or ▁fib ro bl asts , ▁this ▁cy tok ine ▁is ▁produced ▁as ▁a ▁m ature ▁protein ▁mainly ▁by ▁d end rit ic ▁cells , ▁mon ocy tes ▁and ▁macro ph ages . ▁This ▁dis cre p ancy ▁between ▁the ▁wide ▁appearance ▁of ▁IL - 1 5 ▁m R NA ▁and ▁limited ▁production ▁of ▁protein ▁might ▁be ▁explained ▁by ▁the ▁presence ▁of ▁the ▁twelve ▁in ▁humans ▁and ▁five ▁in ▁m ice ▁up stream ▁initi ating ▁cod ons , ▁which ▁can