MEDLINE® (Medical Literature Analysis and Retrieval System Online) is the U.S. National Library of Medicine's® (NLM) premier bibliographic database that contains approximately 12 million references to journal articles in life sciences with a concentration on biomedicine. Articles cover topics in medicine, nursing, dentistry, veterinary medicine, the health care system, pre-clinical sciences, and much more. MEDLINE uses MeSH (Medical Subject Headings) indexing with tree, tree hierarchy, subheadings and explosion capabilities to search citations from over 4,800 current biomedical journals. Included are citations from Index Medicus, International Nursing Index, Index to Dental Literature, HISTLINE, SPACELINE, PREMEDLINE®, AIDSLINE®, BIOETHICSLINE®, and HealthSTAR®.