A targeting construct was designed to insert a loxP and frt-flanked neomycin (neo) resistance cassette downstream of exon 4 of the opioid receptor, mu 1 (Oprm1) gene. An A to G point mutation was introduced in exon 4, resulting in the amino acid mutation T394A. Flp-mediated recombination removed the FRT-flanked neomycin cassette.