PubMed contains over 20 million citations for biomedical literature from the MEDLINE database, life science journals, and online books. PubMed’s citations and abstracts cover the fields of medicine, nursing, dentistry, veterinary medicine, the health care system, and pre-clinical sciences. PubMed also provides access to additional relevant websites and links to the other NCBI (National Center for Biotechnology Information) molecular biology resources. PubMed is a free resource that is developed and maintained by the NCBI, at the National Library of Medicine (NLM), located at the National Institutes of Health (NIH).