PubMed provides access to MEDLINE, a database of citations covering the fields of medicine, nursing, dentistry, veterinary medicine, the health care system, and preclinical sciences. Other services within PubMed include the Journal Browser, MeSH (Medical Subject Heading) Browser, Citation Matcher, and Clinical Queries. A user-friendly online tutorial is available at the site. PubMed also provides access to molecular biology databases in the National Center for Biotechnology Information s (NCBI) Entrez retrieval system with components for searching DNA and protein sequences, 3-D protein structure data, population study data sets, and assemblies of complete genomes. 1946-present.