comment "Definition: An xref that defines a reference to a publication such as a book, journal article, web page, or software manual.
Usage: The reference may or may not be in a database, although references to PubMed are preferred when possible. The publication should make a direct reference to the instance it is attached to. Publication xrefs should make use of PubMed IDs wherever possible. The DB property of an xref to an entry in PubMed should use the string "PubMed" and not "MEDLINE".
Examples: PubMed:10234245" (string)