The following elements were inserted into intron 1: a loxP site, a neomycin resistance gene cassette, a STOP cassette (containing SV40 poly(A) signal sequence), a second loxP site, sequence for N-terminal FLAG and HA tags, the rat GlcNAc transferase cDNA, an FRT site, IRES sequence, the EGFP fluorescent marker gene, a second FRT site, and bovine growth hormone gene poly(A) signal sequence. The inserted genes are only expressed after cre-mediated deletion of the STOP cassette. (J:283905)