Narcolepsy: Recurrent, uncontrollable, brief episodes of sleep. Natural Killer (NK) Cells: Cells capable of mediating cytotoxic reaction without prior sensitization against the target. NK cells are small lymphocytes. Their cytotoxic activity is not antibody-dependent. NK cells can lyse a wide variety of tumor cells and other cell types. Negative feedback: This occurs where the products of a process can act at an earlier stage in the process to inhibit their own formation. Neoplasia: Formation of abnormal growth, specifically a new growth of tissue that is uncontrolled. Neoplasia could be both benign and malignant. Nerve Fascicle: A small bundle or cluster of nerve fiber. Neural Crest: A specific cellular band present during very early development that gives origin to nerve cells and many other structures. Neuroblastoma: Tumors of nervous system origin affecting mostly infants and children. Neuroendocrine: Pertaining to the anatomical and functional relationship between the nervous system and the endocrine system. Neuron: Any of the conducting cells of the nervous system. A typical neuron consists of a cell body, containing the nucleus and cytoplasm, several short radiating processes known as dendrites, and one long process, the axon. Neurotransmitters: Endogenous signaling molecules that alter the behaviour of neurons or effector cells. Neurotransmitter is used here in its most general sense, including not only messengers that act directly to regulate ion channels, but also those that act through second messenger systems, and those that act at a distance from their site of release. Included are neuromodulators, neuroregulators, neuromediators, and neurohumors, whether or not acting at synapses. Neutron: An electrically neutral or uncharged particle of matter existing along with protons in the atoms of all elements except the mass 1 isotope of hydrogen. Neutropenia: Decrease in the number of neutrophilic (stainable by neutral dyes) white blood cells. Neutrophils: White blood cells stainable with neutral stains. Nevi: A circumscribed stable malformation of the skin and occasionally of the oral mucosa. Nitrogen Mustard: A group of alkylating agents derived from mustard gas, with the sulfur replaced by nitrogen. They were formerly used as toxicants and vesicants, but now function as antineoplastic agent. These compounds are also powerful mutagens, teratogens, immunosuppressants, and carcinogens. Nitrosamine: Any of a group of N-nitroso derivatives of secondary amines (R2N--NO), formed by combining of nitrates with amines. Nitroso compound: Indicating the presence of the group -N:O (Nitrogen:Oxygen). N-myc: Oncogene, related to myc gene, found in neuroblastomas. Non-cycling cells: Cells those are not passing through cell cycle. Non-Hodgkin's Lymphoma: A heterogeneous group of lymphomas arising from the lymphoid components of the immune system. The clinical picture is broadly similar to that of Hodgkin's disease. Reed-Sternberg cells common in Hodgkin's disease is absent in these lymphomas. Nosocomial: Pertaining to or originating in the hospital. Nosologic: Pertaining to the classification of disease. Nucleoprotein: A substance composed of a simple basic protein, usually a histone or protamine, combined with a nucleic acid. Nucleoside: Purine or pyrimidine base linked glycosidically to ribose or deoxyribose, but lacking the phosphate residues that would make it a nucleotide. Nucleosome: Complexes of histone, protein and DNA in higher organisms (eukaryotes). Under the electron microscope the nucleosomes are visible as beadlike bodies on a string of DNA.. In the diagram shown below, on the right is shown "the beads on the string" where the blue rope is the DNA double helix, and the yellow stuffing the histones; on the left is the crystal structure of nucleosome; the two double helical DNAs appear in blue and gold, the histones as spirals in different color. Nucleotide: A compound formed by covalent binding of a purine or pyrimidine base, a pentose sugar, and a phosphate. For example, dAMP. Nucleus: A spheroid membranous body within a cell that contains the genetic material, the DNA, of the organism. See diagram of Cell.