n_domain$xref
#&gt;          namespace              name
#&gt; 1 pubchem.compound  PubChem-compound
#&gt; 2           pubmed            PubMed
#&gt; 3               so Sequence Ontology
#&gt; 4         taxonomy          Taxonomy
#&gt;                                             ids              access_time
#&gt; 1                                      67505836 2017-01-20T09:40:17-0500
#&gt; 2                                      26508693 2017-01-21T09:40:17-0500
#&gt; 3 SO:000002, SO:0000694, SO:0000667, SO: