Your Query returns 19 Accession Numbers:

Query TermAccessionNCBI TaxonidSynonymsPhenotype (OMIM) *
oprm1AJ5195359796
oprm1NM_1317077955or2    ZFOR2   
Oprm1NW_04755010116
Oprm1NM_01307110116MORA   
OPRM1AY2922909606C17T    600018
OPRM1AY2922919606C17T    600018
OPRM1AF5479979606600018
OPRM1AY5210289606600018
OPRM1AH0067059606600018
OPRM1AF0245159606600018
OPRM1AF0245169606600018
OPRM1AF0245179606600018
OPRM1AL1364449606600018
OPRM1NM_0009149606OPRM    MOR-1    600018
OPRM1NM_1744089913
OPRM1NT_0257419606600018
OPRM1AY3648909606600018
OPRM1AY3642309606600018
OPRM1AY5877649606600018

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* OMIM link based on symbol match for human and mouse.

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Text2Knowledge ... Yong Huang 2002