The targeting construct inserted into the 3' UTR downstream of the translation stop codon contains a loxP site, the bovine growth hormone polyadenylation sequence, 2 tandem copies of the 1.2-kb 5' insulator sequence from chicken beta-globin gene, neomycin resistance cassette, loxP site and 80 bp of AU/U-rich element of the mouse Fos gene. These modifications stabilize the transcript and result in a 3-fold increase in transcript expression. (J:201446)