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CXBK mice, recombinant inbred mice derived from C57BL/6By and BALB/cBy progenitors, display reduced morphine-induced analgesia. Earlier we reported that CXBK mice expressed a reduced amount of the major transcript, MOR-1 mRNA, of the mu-opioid receptor gene. The CXBK MOR-1 mRNA contains a normal coding region and an abnormally long untranslated region.To identify the nucleotide-sequence difference between the CXBK MOR-1 mRNA and that of the progenitors, we first characterized the 3′ untranslated region of the MOR-1 mRNA, which was largely unknown. A 3′ rapid amplification of cDNA ends-PCR analysis revealed that the 3′ untranslated region of the C57BL/6By MOR-1 mRNA was 10 181 nucleotides transcribed from an exon. Next, we compared the MOR-1 genes in C57BL/6By, CXBK, and BALB/cBy mice, and found a 5293 nucleotide insertion only in CXBK mice. The inserted sequence was a variant of the intracisternal A-particle elements that exist in the mouse genome at approximately 1000 sites. Reverse transcription–PCR analyses revealed that the intracisternal A-particle element was transcribed as a part of the CXBK MOR-1 mRNA. No other differences were found in the MOR-1 mRNA between CXBK and BALB/cBy mice, whereas 100 nucleotides differed between C57BL/6By and CXBK mice aside from the intracisternal A-particle insertion. Finally, CXBK mice displayed reduced morphine responses compared with BALB/cBy mice.Our data suggest that differences in the MOR-1 3′ untranslated region appear to cause the CXBK phenotype. This genetic mechanism underlying the CXBK phenotype may provide good insight into the possible genetic mechanisms underlying individual differences in opioid sensitivity in humans.