The following elements were inserted into intron 1: a CAG promoter, a loxP site, a neomycin resistance gene cassette, a STOP cassette, a second loxP site, the S. cerevisiae NDI1 gene, an FRT site, an IRES, the EGFP fluorescent marker gene, a second FRT site, the WPRE enhancer and poly(A) signal sequences. This allele will express the inserted gene only after cre-mediated removal of the STOP cassette. (J:296588)