CD21-cre Tg(Cr2-cre)3Cgn O CD21-cre3a Cr2<tm1(cre)Cgn> CD21-cre|Cr2<tm1(cre)Cgn>|CD21-cre3a MGI:3708319 http://identifiers.org/mgd/MGI:3708319 transgene Transgene transgene insertion 3, University of Cologne