Your Query returns 2 Accession Numbers:

Query TermAccessionNCBI TaxonidSynonymsPhenotype (OMIM) *
MGC56642BC0490567955RPS21    zgc:56642   
MGC56642NM_2011917955RPS21    zgc:56642   

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

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Text2Knowledge ... Yong Huang 2002