A loxP-flanked STOP cassette (containing neomycin gene), GFP, and myc tag fused to the N-terminus of Ago2 coding sequence under the control of the CAG promoter were inserted into the locus. Only upon cre-mediated recombination to remove the STOP cassette is GFP and myc-tagged Ago2 expressed. GFP expression indicates expression of Ago2 and the myc epitotpe tag allows for affinity purification of associated miRNAs. (J:179351)