PMID (PubMed ID) is the unique identifier for a publication's abstract in PubMed.
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A DOI (Digital Object Identifier) is a publisher’s unique identifier for a publication. The prefix http://dx.doi.org/ is presumed. Publishers may charge for access.
Fung KW, Bodenreider O.Knowledge representation and ontologies. Richesson RL, Andrews JE (Eds.). Clinical Research Informatics, Health Informatics, DOI 10.1007/978-1-84882-448-5_14, Springer-Verlag London Limited Publishing 2012.