Nucleotide: A nucleic acid unit composed of a five carbon sugar joined to a phosphate group and a nitrogen base.

National Center for Biotechnology Information (NCBI): A unit of the National Library of Medicine (NLM), National Institutes of Health (NIH).

National Institutes of Health (NIH): An agency of the U.S. Department of Health and Human Services, Public Health Service; supports research and training to improve health.

National Library of Medicine (NLM): Part of the National Institutes of Health (NIH). Includes NCBI.

Noncoding RNA: An RNA molecule that does not code for a protein.

Nucleic acid: The term first used to describe the acidic chemical compound isolated from the nuclei of eukaryotic cells. Now used specifically to describe a polymeric molecule comprising nucleotide monomers, such as DNA and RNA.