X 2 ▁alternative ▁sp lic ing ▁region ▁lies ▁in ▁the ▁ext rac ell ular ▁domain ▁and ▁al ters ▁the ▁lig and ▁binding ▁site ; ▁specifically , ▁the ▁conser ved ▁hom ology ▁repeat ▁domains ▁ 3 ▁and ▁ 4 . ▁The ▁first ▁identified ▁human ▁trans cript ▁contains ▁ext rac ell ular ▁and ▁cy top las mic ▁domains ▁corresponding ▁to ▁the ▁mouse ▁X 2 ▁and ▁B ▁variants , ▁respectively . ▁A ▁unique ▁ext rac ell ular ▁sp lice ▁variant ▁was ▁also ▁identified ▁in ▁human . ▁The ▁differential ly ▁sp lic ed ▁variants ▁detected ▁in ▁rod ents ▁have ▁also ▁been ▁detected ▁in ▁humans . ▁Major ▁cy top las mic , ▁development ally ▁reg ulated ▁variants , ▁alpha - 7 A ▁and ▁alpha - 7 B , ▁as ▁well ▁as ▁ext rac ell ular ▁variants , ▁X 1 ▁and ▁X 2 ▁were ▁identified ▁in ▁humans . ▁Moreover , ▁the ▁D ▁variant , ▁but ▁not ▁the ▁C ▁variant ▁was ▁detected ▁in ▁humans . ▁ ▁Al pha - 7 ▁integr in ▁is ▁highly ▁expressed ▁in ▁stri ated ▁mus cle , ▁namely ▁ske let al ▁and ▁card iac ▁mus cle , ▁and ▁functions ▁as ▁the ▁major ▁l amin in - binding ▁integr in . ▁It ▁was ▁later ▁shown ▁that ▁alpha - 7 ▁integr in ▁is ▁also ▁highly ▁expressed ▁in ▁smooth ▁mus cle . ▁The ▁two ▁major ▁sp lice ▁variants ▁of ▁alpha - 7 ▁integr in ▁appear ▁to ▁have ▁development ally ▁reg ulated ▁expression ; ▁alpha - 7 A ▁integr in ▁is ▁expressed ▁sole ly ▁in ▁ske let al ▁mus cle