Exon 11 and part of the adjacent intron, which are located about 30 kb upstream of exon 1, were replaced with an IRES/tau/lacZ-loxP/tk-neo/loxP cassette via homologous recombination. RT-PCR analysis confirmed the absence of transcripts that include exon 11 and indicated about a 25% decrease in overall Oprm1 RNA expression. Receptor binding assays indicated a 21% decrease in binding site density. (J:147079)