The beta-galactosidase (lacZ) neomycin gene cassette, consisting of a lox71 site, lacZ, frt site, neomycin gene, frt site, and loxP site, was inserted into the 5'UTR sequence of exon 1 via homologous recombination, replacing a portion of exon 1 which contains the translational initiation site. Western blot analysis confirmed absence of protein in brain and liver. (J:238086)