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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.
Tse T, Williams RJ, Zarin DA.Update on Registration of Clinical Trials in ClinicalTrials.gov. Chest. 2009 Jul;136(1):304-5. doi: 10.1378/chest.09-1219.
Beaudoin DE, Longo N, Logan RA, Jones JP, Mitchell JA.Using information prescriptions to refer patients with metabolic conditions to the Genetics Home Reference website. J Med Libr Assoc. 2011 Jan;99(1):70-6. doi: 10.3163/1536-5050.99.1.012.