PubMed comprises more than 37 million citations for biomedical literature from MEDLINE, life science journals, and online books. Citations may include links to full-text content from PubMed Central and publisher web sites. MEDLINE (Medical Literature Analysis and Retrieval System Online) is the largest component of PubMed and consists primarily of journal citations; articles indexed with MeSH (Medical Subject Headings) and curated with funding, genetic, chemical and other metadata.