Na/K-ATPase pump  Primary active transport protein that splits ATP and releases energy used to transport sodium out of cell and potassium in.

Natural killer (NK) cells  A group of lymphocytes which have the intrinsic ability to recognize and destroy some virally infected cells and some tumor cells by binding nonspecifically to virus-infected and cancer cells. No prior exposure to the antigen is required.

Neoplasm  Cancerous tissue

Nernst equation  Mathematical expression that defines the electric potential E across a membrane as directly proportional to the logarithm of the ratio of the ion concentrations on either side of the membrane and inversely proportional to the valency of the ions.

Neuroeffector junction  Synapse between a neuron and muscle or gland cell.

Neurohormone  Chemical messenger that is released by a neuron and travels in bloodstream to its target cell.

Neuropeptide  A peptide secreted by neurons that functions as a signaling molecule either at a synapse or elsewhere. These molecules have diverse, often long-lived effects in contrast to neurotransmitters.

Neurotransmitter  Extracellular signaling molecule that is released by the presynaptic neuron at a chemical synapse and relays the signal to the postsynaptic cell. A neurotransmitter can elicit either an excitatory or inhibitory response, but the response, but the response is determined by the receptor activated by the neurotransmitter not by the chemical nature of the neurotransmitter. Examples include acetylcholine, dopamine, GABA (γ-aminobutyric acid), and serotonin.

Nick A single-strand break, involving the absence of one or more nucleotides, in a double-stranded DNA molecule.

Nick translation The repair of a nick with DNA polymerase I, usually to introduce labeled nucleotides into a DNA molecule.

Nicotinamide adenine dinucleotide (NAD⁺)  Coenzyme derived from the B vitamin niacin that transfers hydrogen from one substrate to another.

Neoplastic A new, unnecessary proliferation of cells giving rise to a tumor.

Neoplastic transformation The conversion of a nontumorigenic cell into a tumorigenic cell.

Northern translation The repair of a nick with DNA polymerase I, usually to introduce labeled nucleotides into a DNA molecule.

Nothern transfer A technique for transferring bands of RNA from an agarose gel to a nitrocellulose or similar membrane.

Nude mouse  A genetically athymic mouse which also carries a closely linked gene producing a defect in hair production.

Nucleic acid hybridization Formation of a double-stranded molecule by base-pairing between complementary or homologous polynucleotides.

Nucleolus  Large structure in the nucleus of eukaryotic cells where rRNA synthesis and processing occurs and ribosome subunits are assembled.

Nucleosome  Small structural unit of chromatin consisting of a disk-shaped core of histone proteins around which a ~146-bp segment of DNA is wrapped.