A cassette encoding Cre:GFP (a Myc-tag and nuclear localization signals at the N-terminus of Cre recombinase, which is fused to green fluorescent protein followed by a SV40 polyadenylation sequence) was inserted just 5' of the initiation codon in the first coding exon of the Oprm1 gene replacing the exon 1 via homologous recombination. The neomycin cassette was excised of through crosses with animals broadly expressing Flp recombinase. (J:314358, J:325015)