A construct, consisting of a loxP site, the 3' UTR from Fos, two copies of a transcriptional insulator from the chicken beta globin gene, a neo cassette, a second loxP site and the 3' UTR from bovine GH1, replaced the 3' UTR of the gene. The mRNA produced by this allele is unstable and results in a lower level of expression compared to wild-type littermates. (J:194237)